What do you mean by a "royal rumble"?
1 on 1 is a stomp fest in favor of Gerard, as no one in the verse can even keep pace with him, let alone tank one of his hits. If it is the entire verse against him (especially with prep), he may fall prey to one of the hax.
For example, if Kizaru sent up a large beam of light, it will make Gerard's shadow extend a distance, and if it is cut by Moriah, he will be KO'd just like everyone else who loses their shadow to him, and if the sun is out, Gerard will pretty much be erased, or if that's not the case, Law would have an opportunity to cut him to bits and attach his entire body to a wall and they all lock him up. Perona's negative hollow temporarily slowing him down would also be a factor. I doubt he'd simply avoid it if he is in the heat of battle against the entire verse at once.
Also, Kuma's hax may also factor in. He'd remove the damage from all of the opponents Gerard has slain, and lure him into the giant pink bubble, potentially killing him, if it can bypass his Regenerationn. Doflamingo's parasito may also factor in if he can impale Gerard's neck with one of his threads.
Law's body swap happening is an instant RIP for Gerard, if that manages to hit.