GLib is used as a basic platform abstraction library. It doesn’t provide any direct GUI functionality.
To quote the GLib Reference Manual:
GLib provides the core application building blocks for libraries and applications written in C. It provides the core object system used in GNOME, the main loop implementation, and a large set of utility functions for strings and common data structures.
GLib contains lots of useful things for platform independent development. See https://developer.gnome.org/glib/ for details about GLib.