If you had a Trac account then you likely already have a GitLab account as a result of the Trac import. This account will have a username of the form trac-$TRAC_USERNAME. You can reset your password with the Forgot your password? dialogue. Once you have gained access to this account you may change your username to remove the trac- prefix
If you do not have a Trac account (or would prefer to start from scratch with a new account) you can log in using your GitHub account or create a new GitLab account.
As a last resort, please contact @bgamari with your email address.
Most of the tickets here were carried over from GHC's previous Trac installation. See the wiki for details about this migration.