Hi @Andy, this isn't supported right now as:
Adding more sign-in methods beyond GitLab increases the difficulty for 3rd party app developers to build extensions for Mattermost. Keeping auth simple makes it easier to build 3rd party integrations, which are rapidly growing.
There's a great deal of time and energy that goes into supporting just GitLab sign-in, and the support team isn't keen to add more sign-in methods that would put more demands on their time and lower the quality of support overall
There's a little extra work your team needs to do create an account, but it should pay-off in terms of how many more integrations you can use and build with a simple design.