The GTK client supports currently only GTK±2. The idea is to add first optional support to GTK±3 via pygtkcompat, for that the some part of the code must be rewritten in a compatible way for GTK±2 and GTK±3. Bonus point is to support Python3 via 2to3.
Current work in progress: Issue 5387: Add support to GTK+ 3.0 - Tryton issue tracker
Knowledge in Python and GTK+