Glib is used as a basic platform abstraction library, it's not related to GUI things.

To quote the Glib documentation: “GLib is a general-purpose utility library, which provides many useful data types, macros, type conversions, string utilities, file utilities, a main loop abstraction, and so on. It works on many UNIX-like platforms, Windows, OS/2 and BeOS. GLib is released under the GNU Library General Public License (GNU LGPL).”

GLib contains lot's of useful things for platform independent development. See https://developer.gnome.org/doc/API/2.0/glib/index.html for details about GLib.