Before reporting a bug, make sure it really is a bug, not a simple misunderstanding or a misconfiguration. Please check the manual and this FAQ. You may also ask for help on the firstname.lastname@example.org mailing list (you may want to search the list archive first; it is available from http://lists.gnu.org/mailman/listinfo/help-gnats).
If it is a bug, please report it via the bug tracking system. It resides at http://bugs.gnu.org/cgi-bin/gnatsweb.pl?database=gnats. This is a common bug database for gnats, Gnatsweb and TkGnats. (And, yes, of course: It uses gnats and Gnatsweb.)
When you report problems concerning gnats itself, please do not forget to provide especially the following information:
Providing this information in the initial report avoids further unnecessary communication, saves the limited resources of the developers (keep in mind that they are working on gnats and friends in their spare time) and helps to track down and fix the problem soon.