We have proof Gunko was on God Valley, now, so, I think what happened is simple:
Haki transends all according to Kaido, and CoC is the strongest form of it. The way to break through Domi Reversi is simple: Haki.
Imu needs to break your Will/Haki, in order to Domi Reversi you. Imu didn't turn Brogy at first, Imu shot his hand off. Why? To break Brogy's Will/disrupt his Haki.
Haki CAN resist Domi Reversi, but only if you're actively using it, and in the proper mental state.
This is similar to Law's Haki breaking through Doc Q's Fruit: in order to break through Domi Reversi, you have to burn through a good amount of CoC, possibly actively. It's possible that resisting Domi Reversi with CoC Haki causes Imu to burn through your Haki faster.
We've seen Katakuri's emotional state disrupt his ability to use his Future Sight; Katakuri have proved that Haki is directly tied to your mental state. Which makes sense: Haki is Will Power. Inherited Will.
Like Katakuri, once Harald and Rocks weaknesses were revealed, their Haki faltered; this is when Imu hits you with the horns.
But, Imu has had no problem allowing Kaido and Linlin to reign in the New World for years: Imu is fine making use of others power, and biding their time to do so. Look at Imu taking over Elbaph:
This is NOT the first time Imu's tried. Domi Reversing Harald was Imu's attempt to take over Elbaph, but Loki ruined it.
Rocks watched GunkImu kill his wife, and, completely lost himself in rage. The Domi Reversi Giants we've seen retain their base personality, but become "twisted" evil versions of themselves: it's also possible that rage is needed to trigger Domi Reversi. Bloodlust.
Rocks then became a Demon, who Garp and Roger fought. Problem? GunkImu's still there:
Thankfully, Harald goes to God Valley: Harald was shown protecting non-WG people by attacking WG forces, hurting his own reputation/chance at joining the WG. Harald will sneak away with a few elite guards (and maybe a Loki hiding aboard), and sail to God Valley to help Rocks' family.
When Harald arrives, he'll find Garp and Roger getting beaten by GunkImu + Domi Rocks.
Harald fights Rocks... and puts his friend down for good.
Roger and Garp fight GunkImu
Due to the massive CoC fest this would be, pretty much no one else around the island, besides other Top Tiers, would have no idea what's happening; they'd get blown away, or faint.
Once Rocks becomes Domi Reversi'd, guess who's going to get blamed for everyone turning into Demons? Yep: Rocks.
To the Marines, Garp saved them from Rocks turning an entire island into a demon infested hellhole.
99% of Marines have NO IDEA who Imu is: Vice Admirals at Egghead didn't even know who the Gorosei were when they fought in their full Zoan forms.
Rocks reputation is GOD AWFUL: if everyone around them started turning into Demons, guess what? The Marines are going to think it's him, duh.
Harald will be forced to fully put down his friend, Rocks.
Roger and Garp do some Hakai-esque attack to knock Imu/Gunko back to Marijoa.
Elbaph is given provisional status as a prospective WG nation: but, they want Harald's army to serve the Celestial Dragons as personal troops.
Imu tricks Harald into allowing the God's Knights into the country, so they could betray him/Domi Reversi him and take over Elbaph
We know from Sommers discussing Shamrock visiting Harald's Grave that Shamrock knew Harald. I believe the Abyss can travel to where you've been, similar to Kuma's Fruit.
Garling was sent with Shamrock to negotiate with Harald... Harald, Shanks, and Shamrock, also met at the Reverie, which Shanks discusses when bathing with Gyaban.
Eventually, Garling murders Ida, so Imu can break Harald's Will and turn him via Domi Reversi. The God's Knights pay off a Giant to lock the doors, and steal Elbaph's Fruit that Rocks wanted.
Harald contacts Ida through Den Den Mushi: while they can't live together in the castle, Harald would always call her to talk with her, IMO.
Harald gets a call from a dying Ida, saying she's been murdered, and that she loves him.
Realizing the WG has killed Ida, his wife, just like they killed Rocks' Wife, Harald loses himself when Garling and GunkImu arrive via the Abyss: Garling and GunkImu Domi Reversi Harald, and flee before Jarul and Loki arrive.
Jarul holds Harald down: Loki stabs Jarul in the head, and disrupts his memory.
And, so, Rocks and Harald both died, having their love twisted into bitter rage by the Demon King, Imu.
Harald, benevolent King and Hero of Elbaph, chose the World Government's lies over his friend Rocks' truths: and they both paid dearly.
After killing his father, and realizing Jarul's memory loss means he has no witnesses, Loki flees Gyaban and Shanks, and tracks down the traitor who stole the Fruit: Loki murders them, then eats the Fruit himself, to keep the WG from ever getting it.
Now having eaten the Fruit, Elbaph believes that Loki killed Harald to steal the Fruit: Shanks and Gyaban fight Loki, but he escapes via his Devil Fruit's Abilities.
14 Years Later, it's up to one man, who looked up to both Rocks and Harald as Heroes, to save their legacy:
The scumlord who killed Rocks and Harald is on Elbaph, right now
IT'S CLOBBERING TIME, LOKI, LET'S GET IT
Also, if Garp has met Imu... I think we'll see Teach and Garp have some very interesting conversations, about the Celestial Dragons who chain and enslave the world of Blue Planet
Imu Domi Reversi'd God Valley and Rocks
Haki transends all according to Kaido, and CoC is the strongest form of it. The way to break through Domi Reversi is simple: Haki.
Imu needs to break your Will/Haki, in order to Domi Reversi you. Imu didn't turn Brogy at first, Imu shot his hand off. Why? To break Brogy's Will/disrupt his Haki.
Haki CAN resist Domi Reversi, but only if you're actively using it, and in the proper mental state.
This is similar to Law's Haki breaking through Doc Q's Fruit: in order to break through Domi Reversi, you have to burn through a good amount of CoC, possibly actively. It's possible that resisting Domi Reversi with CoC Haki causes Imu to burn through your Haki faster.
We've seen Katakuri's emotional state disrupt his ability to use his Future Sight; Katakuri have proved that Haki is directly tied to your mental state. Which makes sense: Haki is Will Power. Inherited Will.
Like Katakuri, once Harald and Rocks weaknesses were revealed, their Haki faltered; this is when Imu hits you with the horns.
Imu Murders Rocks' Wife and Harald's Wife Ida
Rocks meets Imu by entering the Flower Chamber and pointing a gun at Imu. Imu is a crazy narcissist: Imu is NOT letting that slide. The first time it happened in history? Oooooh, Imu's mad.
But, Imu has had no problem allowing Kaido and Linlin to reign in the New World for years: Imu is fine making use of others power, and biding their time to do so. Look at Imu taking over Elbaph:
This is NOT the first time Imu's tried. Domi Reversing Harald was Imu's attempt to take over Elbaph, but Loki ruined it.
Rocks watched GunkImu kill his wife, and, completely lost himself in rage. The Domi Reversi Giants we've seen retain their base personality, but become "twisted" evil versions of themselves: it's also possible that rage is needed to trigger Domi Reversi. Bloodlust.
Rocks then became a Demon, who Garp and Roger fought. Problem? GunkImu's still there:
Garp + Roger VS GunkImu + Domi Rocks
Listen, Garp and Roger are strong, but, yikes.
Thankfully, Harald goes to God Valley: Harald was shown protecting non-WG people by attacking WG forces, hurting his own reputation/chance at joining the WG. Harald will sneak away with a few elite guards (and maybe a Loki hiding aboard), and sail to God Valley to help Rocks' family.
When Harald arrives, he'll find Garp and Roger getting beaten by GunkImu + Domi Rocks.
Harald fights Rocks... and puts his friend down for good.
Roger and Garp fight GunkImu
Due to the massive CoC fest this would be, pretty much no one else around the island, besides other Top Tiers, would have no idea what's happening; they'd get blown away, or faint.
The False Demon King, Rocks
The Marines are going to have NO idea why everyone is turning into Demons.
Once Rocks becomes Domi Reversi'd, guess who's going to get blamed for everyone turning into Demons? Yep: Rocks.
To the Marines, Garp saved them from Rocks turning an entire island into a demon infested hellhole.
99% of Marines have NO IDEA who Imu is: Vice Admirals at Egghead didn't even know who the Gorosei were when they fought in their full Zoan forms.
Rocks reputation is GOD AWFUL: if everyone around them started turning into Demons, guess what? The Marines are going to think it's him, duh.
Harald will be forced to fully put down his friend, Rocks.
Roger and Garp do some Hakai-esque attack to knock Imu/Gunko back to Marijoa.
Harald's Mirrored Fate
Harald rips his horns out over the Domi Reversi incident, now believing the Ancient Oni/Ancient Giants descended from actual Demons, or are tied to them.
Elbaph is given provisional status as a prospective WG nation: but, they want Harald's army to serve the Celestial Dragons as personal troops.
Imu tricks Harald into allowing the God's Knights into the country, so they could betray him/Domi Reversi him and take over Elbaph
We know from Sommers discussing Shamrock visiting Harald's Grave that Shamrock knew Harald. I believe the Abyss can travel to where you've been, similar to Kuma's Fruit.
Garling was sent with Shamrock to negotiate with Harald... Harald, Shanks, and Shamrock, also met at the Reverie, which Shanks discusses when bathing with Gyaban.
Eventually, Garling murders Ida, so Imu can break Harald's Will and turn him via Domi Reversi. The God's Knights pay off a Giant to lock the doors, and steal Elbaph's Fruit that Rocks wanted.
Harald contacts Ida through Den Den Mushi: while they can't live together in the castle, Harald would always call her to talk with her, IMO.
Harald gets a call from a dying Ida, saying she's been murdered, and that she loves him.
Realizing the WG has killed Ida, his wife, just like they killed Rocks' Wife, Harald loses himself when Garling and GunkImu arrive via the Abyss: Garling and GunkImu Domi Reversi Harald, and flee before Jarul and Loki arrive.
Jarul holds Harald down: Loki stabs Jarul in the head, and disrupts his memory.
And, so, Rocks and Harald both died, having their love twisted into bitter rage by the Demon King, Imu.
Harald, benevolent King and Hero of Elbaph, chose the World Government's lies over his friend Rocks' truths: and they both paid dearly.
After killing his father, and realizing Jarul's memory loss means he has no witnesses, Loki flees Gyaban and Shanks, and tracks down the traitor who stole the Fruit: Loki murders them, then eats the Fruit himself, to keep the WG from ever getting it.
Now having eaten the Fruit, Elbaph believes that Loki killed Harald to steal the Fruit: Shanks and Gyaban fight Loki, but he escapes via his Devil Fruit's Abilities.
14 Years Later, it's up to one man, who looked up to both Rocks and Harald as Heroes, to save their legacy:
The scumlord who killed Rocks and Harald is on Elbaph, right now
IT'S CLOBBERING TIME, LOKI, LET'S GET IT
Also, if Garp has met Imu... I think we'll see Teach and Garp have some very interesting conversations, about the Celestial Dragons who chain and enslave the world of Blue Planet