Ack! No, it _IS_ supposed to be GT/Payment (there are two sets of modules - GT::Payment ones, which handle the general payment functionality, and GForum::Payment ones, which integrate that general functionality into GForum). It seems the GT::Payment ones weren't packaged with the original .tar.gz installation file. I've fixed this and uploaded fixed package - although I strongly recommend you remove that symlink before extracting it, or else things will definitely break.
Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com