Is this the only integration you have running? Maybe try setting up a different one to test if that works. If Mattermost works with a different integration the issue could be on the GitLab side or a specific problem with the GitLab integration itself (maybe a change in GitLab broke the integration).
If you don't have the latest version of the integration you might be able to get it here. You can also open an issue there to see if anyone else is having your problem.