Java版i2pの場合、i2ptunnelmgrからAuth stringが作れさうです。間違つてゐたらすみません。
https://geti2p.net/ja/docs/applications/gitlab
> Register a Hostname(Optional)
> (略)
> To do this, open the http://127.0.0.1:7657/i2ptunnelmgr again and click on the “Gitlab Web Service” item you just set up. Scroll to the bottom of the “Edit Server Settings” section and click Registration Authentication. Copy the field that says “Authentication for adding Hostname” and visit stats.i2p to add your hostname.