Here are some of the mailing lists that cover GTK+ and Gnome development topics:
email@example.com is appropriate for questions about using GTK+, reading the GTK+ source code, discussion of GTK+ bugs, and so on.
firstname.lastname@example.org is very high traffic; it is the general Gnome mailing list, for discussion of all things Gnome-related.
email@example.com is the right list for questions about the Gnome libraries and how to write programs with them. It's also appropriate for discussing the development of the libraries themselves, or submitting patches which modify the libraries.
firstname.lastname@example.org is an alternative to gtk-list more focused on application development. Most things appropriate for this list are also appropriate for gtk-list, but gtk-app-devel-list has less traffic if you're concerned about that.
email@example.com is for discussion of developing the glib, GDK, and GTK+ libraries. It is not for discussion of using the libraries to develop applications.
firstname.lastname@example.org carries announcements related to Gnome and Gnome applications. It is relatively low-traffic.
To subscribe to any of these mailing lists, append -request to the list name and send a message with subscribe email@example.com in the subject. For example, I would subscribe to firstname.lastname@example.org by sending a message to email@example.com with subscribe firstname.lastname@example.org in the subject line.
Please, ask any and all questions about GTK+ and Gnome development on one of the mailing lists or on IRC. It is inappropriate to mail the developers privately, unless you have reason to believe they have special knowledge of some particular section of code. You'll almost certainly get a faster and better response from the list anyway.