We are a team of 60+ developers working on the same product and are moving from SVN to Git and GitHub. We have a process in SVN where in individual files are locked and when
Git does not provide any locking functionality, since it is decentralized. However, if you host your code on GitLab Enterprise Edition Premium, you can use the web interface to lock individual files or folders, achieving exactly what you want to do.
If you do not want to host your project on someone else's server (their website), you can also download GitLab and host it on your own webserver.