Simplest way is don’t. Gitlab really does not play nice with proxies. It really should, and why it has a bundled nginx itself is beyond me. It just break stuff. Hey ho.
I don’t really like it but the reverse proxing was just a nightmare and I gave up in the end.
Just run your gitlab install on a different port. Then on your router just port forward to the gitlab port. Do the same with ssh as well.