PortaBase uses the Qt library for GUI widgets, strings, data structures, etc.; porting it to use GTK+ would effectively be a full rewrite, and isn't likely to be done by the original author (me).
Anyway, feel free to poke around the PortaBase documentation and source code and scavenge ideas from it...a lot of the UI design, data schema, and overall code structure could be reused for a database application in any language or widget library.