Having a separate GTK2 module was the original plan. Since it never got carried out, I created a patch which allows the old GTK module to be linked against GTK2, but to avoid removing incentive for actually implementing a real GTK2 module, I did not commit it. :-)
If you want to implement the GTK2 module, go for it! If you also want to implement the compatibility layer once planned, even better. The idea was to write, in Pike code, an emulation of the old 1.2 API using the new 2.x API, to ensure compatibility with old programs even if you don't actually have GTK 1.2 on the machine. If the emulation is good enough, it would probably even be a good idea to use it over the old module on systems with both GTK 1.2 and GTK 2.x, in order to A) reduce compilation time and installation size, and B) unify settings etc, since GTK 1.2 and GTK 2.x have separate configuration files.