Mirroring external repo

A little background.

I have a user with an external gitlab repo with an python application that they are developing. I need to pull/mirror it into the repo I control and add deployment scripting that cannot be exposed to the external repo.

question is how do i manage branches and adding the internal scripting and still accept updates to be pulled from the external repo?

You can setup a pipeline that would run regularly and do all the steps you need to update and push it back to GitLab.