Google is probably a better bet for doing searches. No offence to the operators of GTP, but vBulletin's search function is depressingly bad. You can't even search for 3-character terms. FGT, Ruf, ASL, all unsearchable through vBulletin.
Add site:gtplanet.net/forum to your google search for better results. I'm not positive that the /forum has any effect, but it doesn't cause the search to fail or anything, and it seems to filter the results to those just from the forum. I don't think google's site: operator will understand forumdisplay.php?f=xxx though, so searching specific forums is sadly out.
Anyway, just my $0.02.