Hi @au-ee,
Currently you can trigger a sync by temporarily switching the user's account to Email/Password and then having them switch back to GitLab through Account Settings -> Security -> Sign-in Method. You can reset their account to Email/Password in the System Console by going to the Users list under a team that user is on, and selecting "Switch to Email/Password".
I know this isn't the best solution and we're looking to ways to improve the OAuth synchronization. I've created a ticket here to cause a sync on log in to start towards addressing this issue.