All accounts created since January 2016 use the authentication from https://civicrm.org, this means that you first need to make sure that your password works on that site. If you need to change your password, you can change it there (with the request new password form).
Make sure that your civicrm.org username does not have a space or other special characters.
Usernames are case sensitive (ex: "JohnDoe" is not the same as "johndoe").
You cannot use your email as your username (because of Drupal restrictions on special characters).
Do not use the 'Standard' tab on the Gitlab login page. The "Forgot your password" on that block will not work either.