prometheus connecting to external grafana


First time user of prometheus and therefore struggling to see where I can make the changes required. I have enabled prometheus on gitlab and I can see the port listening, but only locally;

tcp 0 0* LISTEN 6102/prometheus

What I have attempted to do, from some of the documentation I have read is to open that up for my grafana server.

I edit the file
#> vi /var/opt/gitlab/prometheus/prometheus.yml

and in the new target, the config takes, but it doesn’t seem to be listening and grafana is providing me a bad gateway error. Can someone please advise on how to access prometheus to an external grafana box?