Specify a custom private key :

Add a specific configuration for your host in ssh config

Host myname.gitlab.com
    Hostname gitlab.com
    IdentityFile /home/myuser/.ssh/keys.d/id_rsa_git
    IdentitiesOnly yes

Test SSH :

ssh -Tvvv git@myname.gitlab.com

Git command :

git clone git@myname.gitlab.com:yourrepo.git