Gmaster doesn't currently support OAuth to login to GitLab (either at gitlab.com or hosted). Unfortunately, GitLab only allows OAuth login to access its API. This is a temporary workaround to access your GitLab account information from Gmaster until we finally implement OAuth support for GitLab!
The "Personal token" field is the key here. You have two options: either use your unique Private Token from GitLab or create a Personal Access Token.
First of all, you need to have in mind that this Private Token is only useful for API operations, such as creating a new repository or listing your currently available repositories. This means that you'll need to enter your credentials later if you wish to clone one of your repos.