HK pranks involving himself with Reverie is an unusual thing clearly. That's why gorosei said that.
Pranks is the Man Marked by Flames and has the last Road Poneglyph. This mirrors Shanks being Luffy's final test to get to Laugh Tale as the last OG Yonkou to be defeated.
Gunko has the Spin Spin, or Spiral Spiral, Fruit, which she uses to create the whirlpools that sink ships that approach Pranks' black ship. She can control the rotation of objects around her. Similar to choking the wolf, whirlpools cause death by choking through drowning.
Shiryu said the WG had the last Pongeglyph. Pranks' duty, because Shanks helped Roger become King of the Pirates, is to make sure no pirate ever reaches Laugh Tale again. While Shanks wanted Luffy to go to Laugh Tale, Pranks is preventing this by holding the Poneglyph. I believe Pranks was part of the force that returned Otohime to Fish-Men Island after she journeyed to Mary Geoise with Myosgard. Pranks found the Road Poneglyph on Fishmen Island, and stole it.
Pranks isn't supposed to leave the ship he guards the Road Poneglyph on, but Pranks had reason to warn the Gorosei about a Pirate.
My guess is on Teach, and that Pranks believes Blackbeard needs to be stopped. This is why they wanted Loki; to kill Teach. Teach is weak to physical damage, and Loki just looks like a brawler. Considering Teach's massive ties with Shanks in the plotline, I expect Teach and Pranks to have some kind of plotline together as well.
Shiryu mentioning this was also trying them together; Kuzan joining Teach is horrible for Pranks. Gunko's whirlpool trick can't work if Kuzan freezes the fucking ocean. This is why the Man Marked by Flames was brought up during Kuzan's recruitment to the Blackbeard Pirates. Even if I'm wrong about Pranks being the Man Marked by Flames, if they do work for the government, Teach having Kuzan makes defeating them easy. Ain't no way some random new character is going to solo Blackbeard, Kuzan, Shiryu, Augur, Burgess, and the rest at this point.
The WG can't trust Pranks to keep the Poneglyph safe by himself anymore, since Pirate crews are actively moving for it.
Keep in mind the WG has Uranus and the Mother Flame; a Wolf is a small threat. Pranks can threaten Loki's entire island.