I'm guessing this is small because it relies on GObject Inspection.

This is a Gnome feature whereby C libraries are accompanied by an XML description of functions and types. The XML description annotates semantics, not only syntax. Like if a function has a pointer parameter, does it have to be malloced, and does the caller free it or the callee? This information is not possible to determine from parsing C header files.

If you can parse the GObject XML, you can then generate FFI bindings (which can be done in Lisp easily without any textual code generation hacks).