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).

Metakit isn't a graphical toolkit, it's the embedded database library used to manage the data files. Writing such an application now, SQLite would probably be a better choice, but it was missing some pretty important features at the time (like the ability to add columns to an existing table). Metakit does have its advantages (I think it's still a little faster than SQLite, for one thing), but SQLite is under more active development, and people understand SQL-based databases better.

Oh, and PortaBase development is only "mostly dead". I have made some progress on cleaning up the code and making it easier to maintain, but the basic problem is that it got too big to manage with one developer's free time. Resolving packaging issues and minor code differences on two different Zaurus ROMs (which use a very old version of Qt), Windows, Linux PCs, and a mostly-finished Mac port, all translated into several different languages...the release logistics just became a nightmare. I do hope to release another version sometime, but I'm not sure when it'll be...I need to finish writing some tools to streamline the build, testing, and translation process first.

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.