The Problem with an ssh-server behind traefik is that ssh-clients do not send a HostSNI Parameter with the tcp connection because it does not use tls.

traefik community post with the same problem

https://community.traefik.io/t/routing-ssh-traffic-with-traefik-v2/717

But Traefik can only route if it is present.

I have tried myself setting up a git server behind traefik my easiest solution was putting it on another port.

I don't know about gitlab but eg. Gogs (a git server) allows setting a port in the config for ssh cloning so the provided command in the ui uses that.