There are instructions to install gtk-3-fortran under macOS in the wiki: Home · vmagnin/gtk-fortran Wiki · GitHub
As GTK 4 seems to now be available in homebrew (gtk4 — Homebrew Formulae), it should be possible to install gtk-4-fortran and make those new examples work under macOS.
Concerning the license, I just know that since the beginning gtk-fortran is licensed under GNU General Public License version 3, with the additional permissions described in the GCC Runtime Library Exception version 3.1, which says:
When you use GCC to compile a program, GCC may combine portions of certain GCC header files and runtime libraries with the compiled program. The purpose of this Exception is to allow compilation of non-GPL (including proprietary) programs to use, in this way, the header files and runtime libraries covered by this Exception.
But I am not sure it is related to what you say about “dynamically linking against the .so libraries”. My brain is lost amongst technique, law and English… 