Hi @icanogbh,
Thanks for your issue report!
I tested on our GitLab instance and push notifications are working fine for me. The only difference is that this instance is on Mattermost v3.8.0. At the moment, we're looking into fixing the "sign-up with GitLab" option as it's broken but you can sign up using email/password option.
Mattermost v3.8.0 was released yesterday so GitLab should be upgrading to the new version in a couple of days.
Hope this helps?