Evidentially it is possible to make GTK apps work at least somewhat nicely on Windows and Mac since there are a few out there. But it's clearly a ton of work. If you just make a standard GTK app it will be terrible. Most toolkits (e.g. Qt or even Flutter) work much nicer without having to fix them.
That said GTK on Windows and macOS is very meh. You don't choose GTK for making a cross platform app.