Gtk+2 is EOL, time to move on.
Last year the Gtk developers made the 2.x End of Life with the release of 4.0, meaning that it will no longer be supporten or updated. That practically means that distributions may stop shiping the library, and that incompatabilitias with -- say -- GLib/GObject may occur. Porting would also bring native support for the Wayland protocol.
https://blog.gtk.org/2020/12/16/gtk-4-0/
Having to identify what needs to be done to update NetRadient to Gtk+3 or Gtk4.x should be done.
The blocker for upgrading to 3 many a years ago his the lack of a GL surface, but that now exist in both 3 and 4: https://docs.gtk.org/gtk4/class.GLArea.html
The main window must be rewritten to make use of GtkApplication, as in 4 the ability to create a GtkWindow directly is discouraged: https://docs.gtk.org/gtk4/migrating-3to4.html
Just a thought. Gego.