How add ssh key for push mirror repo in EE ?
I’m not sure about EE, but CE (or EE without a specific license) will not allow you to specify the key used for mirroring. Instead, when you configure the mirroring the GitLab server will generate its own key to use and provide you with a link to copy the public key to your clipboard. You can then use the public key to authorize GitLab to access the repository.
Hope this helps.