Nginx as a reverse proxy has been covered on the forum before, this post shows how the external nginx should be configured: 502 Bad Gateway - Unraid | Cloudflare | NGINX - #3 by iwalker
This post shows how gitlab.rb should be configured: 502 Bad Gateway - Unraid | Cloudflare | NGINX - #6 by iwalker
As @dnsmichi has said, it’s most likely you have something configured incorrectly, either the external nginx, or something missing in gitlab.rb.
You will need to adapt ports, SSL certs to your requirements since the examples in the above were written specifically for the guy who was experiencing his problem. That said his was slightly different due to running via docker on unraid, but the way the config is written is effectively the same. The Gitlab documentation also mentions about external proxy and what is required in gitlab.rb as well, but a working example usually helps as shown above.