I don't believe we have an existing authentication plugin for Drupal. Depending on your familiarity with our coding style/software it should only take a few hours of work to get an authentication plugin done. Authentication in gforum starts in GForum::authenticate, so that's a good starting place.
The typical authentication plugin typically overrides most of the methods in GForum/Authenticate.pm. So for example, you might override auth_init to load the drupal config (so you can get access to Drupal database for session data [I'm not sure how session data is stored in Drupal]), then you'll need to override auth_valid_session to grab Drupal's session (cookie, url, etc) and check it against their sessions. You get the drift .
Adrian
The typical authentication plugin typically overrides most of the methods in GForum/Authenticate.pm. So for example, you might override auth_init to load the drupal config (so you can get access to Drupal database for session data [I'm not sure how session data is stored in Drupal]), then you'll need to override auth_valid_session to grab Drupal's session (cookie, url, etc) and check it against their sessions. You get the drift .
Adrian