Recently while working on a freelance project, i came across an interesting scenario where the previous developer has maintained 2 repositories, one in gitlab and the other in heroku.
The gitlab account is a repository to hold the code, and the heroku repository is linked to the build pipeline. i have made changes to the gitlab repository and the changes needs to be made to the master branch of heroku repository.
This is a scenario which i have never encountered before. After some investigation i found out that, you would do this when you want to create a new app from the same base or core (that's meant to be its purpose, but the scenario here is different).
Coming to the solution,
Add the other repository as a remote
git remote add upstream https://www.gitlab.com/something/somrepo.git
(upstream refers to the gitlab repository)
Now pull the changes from master branch of gitlab repository to heroku.
git pull upstream master
Push the changes to the repository , now your heroku branch will have all the changes from the master branch of gitlab.