Before 1.9.20 Gaby used a plug-in mechanism for print plug-ins similar to the ones you can still find for formats, views and actions.
That mechanism meant plug-ins had to be written in C (actually it was theorically possible to use C++ without much troubles) and that was quite stupid since that meant dealing with strings and C is not the ideal language to do that...
So I decided to move from C plug-ins to Python plug-ins (Python was chosen over Perl or any language you might want because the only embedded interpreter in Gaby is actually Python (and it's a nice language!)).
This file should explain you the basics you have to know to write print plug-ins for Gaby.