The Perl bindings to GTK follow the original library fairly closely, so that most documentation you find about writing for GTK in C/C++/C#/Python/Ruby/OCaml etc. might help you. Also, GTK has active mailing lists with typically friendly and quick replies to questions.

Did you see the Reference Manual? It might give you some idea of what it's like to work with the library.

PS: I was only half joking about the OCaml bindings :-)