GTK and Motif are both X/Windows "toolkits", sets of GUI widgets that applications can be built with. GTK is the GIMP ToolKit; it's a modern toolkit. The GNOME desktop uses it, as do many native Linux apps.
Motif is an old-fashioned toolit that looks terrible and is awkward for the user and for the programmer. About the only thing is has going for it is that it's a defacto standard simply by virtue of its age. You won't see a lot of Motif usage on the typical Linux system;
Java's AWT was an exception.
Get the GTK version.