FreeType uses HarfBuzz as a shaper for "improved auto-hinting of OpenType fonts". This is not strictly required for basic FreeType operation, but it's a compile-time choice whether to use it or not.
HarfBuzz doesn't use the GLib common runtime much, but it seems to import the GLib-provided Unicode normalization functions.
(Plus, the additional CLI tools (e.g. 'hb-view' or 'hb-shape') are also written using GLib-provided functions for string manipulation, UTF-8 decoding, and so on. These tools are not required for basic operation, but it's each distribution's decision as to whether include them in the main package or to split into a "-bin" package.)
GLib includes developer tools which are written in Python, such as 'gdbus-codegen' or 'glib-mkenums'. These are not required for basic operation, but it's each distribution's decision as to whether make Python an optional dep, or to split developer tools into a "-dev" package, or to do it the lazy way.
Overall, the three libraries themselves are very reasonable dependencies to have (that is, minus the dependency cycle that's unsolved for 6 years and counting), but forcing the GLib package to depend on Python is your distribution's fault.