You shouldn't use gtk or qt on windows but rather windows forms and on MacOs GTK2 is an absolute pain, but i think QT5 works okish, but here cocoa is the way to go.
On Linux the decision is rather simple. GTK2 is officially considered EOL (end of life) by their developers and there will not be any more patches. It is removed from more and more platforms as default system. With respect to QT, while QT6 was also rolled out last year, QT5 is still considered live and, while the official QT team only provides LTS for commercial customers, the KDE Team publishes regular updates and fixes to QT5. Also as the main platform for KDE, it will be shipped with all KDE platforms for the next few years.
So the decision is pretty simple use QT5, especially if you consider developing commercial software, because using EOL libraries is irresponsible and you can be held liable to any bugs or security issues arising from this decision (as, unless otherwise necessary and/or negotiated with the client, you must provide a service according to the state of the art).