Flatpak has the last version, which is indeed 2.10.6. But maybe you
installed it when it was 2.10.4 and have not updated it yourself since.
Please run `flatpak update` to do so (if you have some desktop with deeper
flatpak support, like a recent GNOME, it could even propose you to update
by itself) and you should get GIMP 2.10.6.
As many distributions, they are late in updating packages, and in
particular keep older versions for stability reasons (even though we think
GIMP 2.10 is stabler, sometimes "stable" means not breaking habits).
In any case, we don't maintain distribution packages. We don't know when
they will update.