To use gitlab or whatever git provider follow theses steps
Make your project as git repository by executing the command
Move to Team Explorer tab and click on changes
Add your commit message and then commit changes.
Select Remote Repository
Copy the HTTPS url of your repository from git provider like gitlab
Add the HTTPS url of the repository
then click publish and visual studio will ask you to enter your credential for that git provider
Sync your repository
Now for any upcoming changes or pull requests you have to commit any changes first and then click on home button and select sync to pull the new commits and push the your commits.