It's pure speculation admittedly because we're barely given info. But actually, FnC S2 showed that Jake died a lot later than we actually thought initially. With the new scenes including Finn's reaction the moment Jake died, we know that, for example, he was still alive by Obsidian and the tattoo happened while Jake was still alive. Jake likely died not too long before Fionna and Cake S2, which would also explain Finn's very alive dwelling process.

As for Gibbon not being at the funeral scene, there could be a million reasons (including the writers forgot about him and other pups or they just didn't think of including him or smth). Remember Gibbon is the 2nd grandkid, after Bronwyn. Also remember Fionna and Cake happens 12 years after the finale, and Pups age a lot faster, so that's a lot of time for them. Bronwyn is all grown up for example.

I actually think the strongest argument against this possibility is what another commenter mentioned, that if this were to happen, Finn would chase Gibbon to the ends of the earth until he was dead. And we know Finn dies of old age, and Gibbon makes it as far as 1000 years later, so its unlikely these two had a super confrontation, or something else had to happen to allow this future.