You can also use the Qt5 backend for Lazarus, AFAIK many people use it instead of Gtk2 despite the latter being the default. One annoyance with that is that you need to either distribute or depend on the qt5pas library which acts as an intermediate between Qt and Free Pascal, but that might not be
that big of a problem since AFAIK it is now available in all major distros.
About Gtk i'm not sure if it is lack of maintainers or simply lack of interest in the Gtk3+ backends, i remember a discussion from not too long ago in the mailing list where people weren't happy with the breakage introduced by Gtk every major versions. It can be very disheartening to spend years building the Gtk2 backend only to have to make a new one for Gtk3 and before that is finished, out comes Gtk4 breaking things yet again.
But IME the Gtk2 backend works fine. At least personally i wouldn't bother with Gtk3 or Gtk4 as these feel like a waste of time now (though personally i did submit a patch recently with fixes for the Gtk1.2 backend[0] that didn't work for some time now :-P but that was mainly for fun - there might be some practical use for it though as all the necessary .so files are like ~2.5MB, so perhaps it could be used for simple GUI apps on Linux to minimize external dependencies).
[0] https://i.imgur.com/OM8bC9i.png