Assuming you can access your “internal” plugin repo on your gitlab server via SSH (i.e., without password), I’m guessing you need to add your gitlab server’s hostkey to your known_hosts.
The thread above has a lot more conceptual info that may be helpful.
Also be sure your ssh-agent has your keys loaded. If relevant ssh keys to connect to your gitlab server don’t appear in output of ssh-add -l, you must ssh-add.