Transmission uses Qt on Windows, either GTK or Qt on Linux, and something AppKit on macOS.

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.

Transmission has Qt and Win32-native GUI clients as an alternative to its GTK client.

I don't think Transmission is a good example.

Transmission only uses GTK on linux.

There is a Qt and CLI client, too.

And a web UI! (My preferred method as I run it on my home server)