It is indeed possible, but it’s not as simple as it should be. In Qovery V3 (new version) it will be much simpler. You have two possibilities at the moment:
Use the same email address for your GitLab and GitHub accounts. Then Qovery will propose you link both accounts.
Sign in with your GitLab account and then go to your organization settings and install the GitHub app. (cf screenshot)
I do have a gitlab account that uses the same custom domain email I use in github. Also, my gitlab account is even connected to my github account and my gmail account as valid login options.
I even tried logging in to the qovery console in incognito mode using my gitlab account but then it starts the onboarding process as if it were a new account and asks me to create a new organization, and if I enter my organization name it says that it already exists.
Seems that in the scenario you proposed you first created the qovery account with gitlab and then linked github, but in my case, my qovery organization was created with my github account and now I seem to be unable to link my gitlab account.