The key is in ~/.ssh/config, here you can give aliases for every user on same host. SSH Key on each GitLab account must be already set up.
Host user1.gitlab.com
Hostname gitlab.com
PreferredAuthentications publickey
IdentityFile ~/.ssh/id_rsa_user1
Host user1.gitlab.com
Hostname gitlab.com
PreferredAuthentications publickey
IdentityFile ~/.ssh/id_rsa_user2
Testing:
$ ssh -T git@user1.gitlab.com Welcome to GitLab, @user1!$ ssh -T git@user2.gitlab.com Welcome to GitLab, @user2!
