Do all user's need to be on the same Pro version?
Not necessarily, though it would definitely be best practice for all users to be on the same version. Why? Well if one collaborator makes a change to a project that is based on functionality implemented in the latest release, that 'thing' may not be supported/available for another user who is working with the project in an older version. So keeping all collaborators on the same version is a must in my opinion.
I'm not familiar with GitLab so I can't speak to that. Currently, if two or more users all have access to a project (.aprx file) in a shared location, the first user who opens it will be able to make and save changes. Any other user who opens the same file will get a [Read Only] version. Read this help doc to understand what gets saved to databases vs. what gets saved to a project: Open a project—ArcGIS Pro | ArcGIS Desktop
You might also want to keep an eye on the progress of 'Projects in the Enterprise' functionality noted in the ArcGIS Pro Roadmap - July 2019
