@alerque
Did you find a solution to your problems. I think that I'm facing the same situation after Mattermost 3.0 Upgrade.
I was able to restore my password and then login in using user and password, but now according to @lfbrock if I want to switch my account back to Gitlab I'm unable because under Sign-in Method there is no gitlab option.
Do you have any news about this issue?
Thanks in advance.