Well, Gtk4 isn't supported (in fact even Gtk3 support is buggy, AFAIK the original maintainer abandoned it after getting tired of Gtk4 breaking the API again - i think there is someone else fixing bugs for Gtk3 right now though but there isn't any Gtk4 code), though from a quick search it can be statically linked if you build it from source yourself (the Gtk developers do not seem to like that though and all comments i saw in a couple of relevant bugtracker posts were along the lines of "do not do that, we don't want to support that"). So assuming a Gtk4 backend is made and you are able to create a static Gtk4 library (together with static versions of all the libraries it needs aside from common stuff like the C library, OpenGL, etc) it should work.
It might be possible to link against Qt6 though (the LCL widgetset of which is more actively developed) but you'd still need to make a static library for both Qt6 and libQt6Pas (a "proxy" library that exposes a C API for Qt6 that LCL can use, currently it is part of the Lazarus project but there was some discussion recently to make it its own separate thing). Personally i've only tried linking statically against libQt6Pas (with the Qt6 libraries being linked dynamically so the program can use the distro libs) so i wont have to carry the libQt6Pas.so[0] (Qt6 is or at least should be available pretty much everywhere nowadays and will be around for years to come).
In general you should be able to link against any static C or C++ library (note that for the latter you also need to link against the C++ libraries).
It might be possible to link against Qt6 though (the LCL widgetset of which is more actively developed) but you'd still need to make a static library for both Qt6 and libQt6Pas (a "proxy" library that exposes a C API for Qt6 that LCL can use, currently it is part of the Lazarus project but there was some discussion recently to make it its own separate thing). Personally i've only tried linking statically against libQt6Pas (with the Qt6 libraries being linked dynamically so the program can use the distro libs) so i wont have to carry the libQt6Pas.so[0] (Qt6 is or at least should be available pretty much everywhere nowadays and will be around for years to come).
In general you should be able to link against any static C or C++ library (note that for the latter you also need to link against the C++ libraries).
[0] https://i.imgur.com/Cw89XtC.png