That isn't an error; just a warning. If you download Gossamer Forum again and perform an "upgrade" of your existing installation, you'll get this fix a few others. In particular, the original 1.2.0 that was posted was missing a few required payment modules.
What is the error? I recommend downloading the gforum-1.2.0.tar.gz file again and reinstalling - it will most likely correct the errors encountered, and contains a couple initial bug fixes that were found (mostly reported in this forum) since the original release.
Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com