You would have to contact Gitlab support directly - they will not deal with it via the forums. Here is the link and open a ticket: Support | GitLab
If you are a free user you will have to wait patiently for them to reply. Although there was something I believe that they don’t reset 2FA now for free accounts, only for paying users, but you can try anyway.
I suggest for the future that you keep a backup of your SSH keys, and if you used 2FA on your phone, Authy is a really good app which allows you to backup your 2FA account in the cloud. Then if you lose your phone, you can install Authy on a new phone and restore the backup - providing that you remember the password you used for the backup!
I have Authy installed on two telephones, so I can always get in to my account. I’ve also even started using Yubikeys as well for 2FA for additional login abilities.