On your gitlab server run gitlab-rails console production
Find your user via user = User.find_by(email: "youroldemail@example.com")
Optionally change the user's email with user.email = "yournewemail@example.com"
Then run user.save!
Get the user's token with user.confirmation_token
https://PutYourGitlabHere/users/confirmation?confirmation_token=PutYourTokenHere