[3v1n0]

Gtk is multi platform.

Being used more by linux doesn't change its nature, and plenty of stuff you never realized use it in many platforms.

[PaulDavisThe1st]

Not entirely true. The resources devoted to making sure the backends for non-Linux platforms are correctly designed and implemented suffer a bit.

I say this as someone using GTK and who was forced to contribute quite a lot of fixes for the macOS backend (in GTK2), because I was one of the few people using it in the way we do on macOS.