GDK is, as I understand it, the layer below GTK+. Sort of like the glue between X and GTK+, it handles stuff like graphics contexts and colormaps and such. Well, drawing, pretty much. It stands for the "GIMP Drawing Kit".

_{this nodeshell rescued by the bored gtk+ developer who can't seem to figure out how to write a simple gdk-based sprite engine that doesn't segfault at the moment rescue team. yeah, we're pretty specialized.}

Also, Microsoft's (former?) name for the DirectX SDK.

Game Development Kit

This was the name for DirectX 1 or 2, I think. Twas actually kinda nifty, for an aspiring game developer, to inspect the sample code, and most of the demos survived at least 3 versions longer...