I think it's a combination of all the above.
Imu probably has the Forest God Fruit. AND Imu likely has strong Haki. AND Imu probably has access to some third power system, the Abyss Mark powers. I think what we're seeing is a combination of all the power systems in this world, so Imu can make some crazy chain combos.
The Forest God Fruit is probably where all the "game" powers are coming from. The "Othello" and "Clue Revolver" and "Onigokko" attacks. I know that doesn't seem very "Forest God"-esque. But, to be fair, none of Luffy's Nika powers seem especially "Sun God"-esque, either. The God Fruits just seem to be overpowered reality warpers. Even the Grimoire could actually be a Dungeons and Dragons style Monster Manual, technically meaning it still falls under the "game" powers. Imu's just the Dungeon Master.
https://en.wikipedia.org/wiki/Reversi
https://cluepedia.fandom.com/wiki/Revolver
https://bujinkansantamonica.blogspot.com/2012/10/onigokko-lets-pretend-we-are-demons.html
https://en.wikipedia.org/wiki/Kagome_Kagome
https://en.wikipedia.org/wiki/Monster_Manual
The Haki powers are pretty obvious. Imu was able to send out a giant Conqueror's Haki blast throughout all of Elbaph. It's likely Imu is using their Haki to strengthen and enhance their other abilities.
And I think some third power system is still on the table. I mean, all of the magic circles could just be explained as a extension of their Devil Fruit, if Imu is using "Dungeon Master" powers. Or some sort of "trading card game" powers, like think of something like Yu-Gi-Oh or Magic the Gathering, where you summon monsters to the field.
But, I think we've gotten enough hints that the Abyss Marks are their own thing, rather than a extension of Imu's God Fruit. Imu's been using their abilities on both organic and inorganic objects, which is uncommon for a Devil Fruit to be able to affect both. Usually it's one or the other. The Abyss Mark symbols stay on the God's Knight's bodies and in King Harald's throne room, even with Imu being thousands of miles away. Usually, with Devil Fruits have some sort of a range. You at least have to stay on the same island with the people you're affecting with your powers. This power doesn't seem to have a range limit. It's also unusual for a Devil Fruit to require a "chant" before the User uses the power. Usually, the power just works without any vocal command, unless it's a sound-based Fruit. Plus, it just has a different "vibe" the rest of Imu's powers. It "feels" like a separate thing to me. So, it COULD all just be the result of a overpowered Devil Fruit. Like Sugar's Devil Fruit on steroids. But, I think we're dealing with something else being used in conjunction with the two power systems we already know about.
As for it being "magic". Well, there's that old quote "Any sufficiently advanced technology is indistinguishable from magic." So, I'm not quite ready to say it's "real" magic. Since it could just be explained with...nanobots or hard-light holograms, or some other sci-fi deus ex machina. But, as for the "spirit" of the question, yes, I think Imu is using something other than Devil Fruits to do what they're doing.