I did commit successfully in my local repository. When I try to do:
git push https://gitlab.com/priceinsight/jmt4manager/compare/develop...2-retrieve-list-us
I also understand that your question was solved, but since I had a different solution and this is the top search result for the error, here's my ten cents...
I simply had the wrong repo string. In the GCP repo viewer they given you a HEAD URL that's NOT the same as the remote URI that you should use. In my case, the HEAD URL provided in the "Links" drop-down in the web console was:
https://source.cloud.google.com/[PROJECT]/[REPO]/+/master:
(of course I removed the "/+/master:" at the end)
Whereas my actual (correct, working) repo URL is:
https://source.developers.google.com/p/[PROJECT]/r/[REPO]
I got the format from this page. Not sure why this isn't one of the options in the web portal, but oh well.
Hope this helps someone else!