Gitlab and Github share the same purpose - git repository management with extensive features like code reviews, issue tracking, activity feeds, wikis, protected branches, etc.
There are some differences between the two services though:
WIP to a merge request title will prevent anyone from merging it. Github on the other hand is way more popular and widely used than GitLab. One of the most used libraries and frameworks are shared there.
Two months ago, a group of open source maintainers wrote an open letter to Github, in the form of a repository, where they issued some of the most needed features Github lacked at that moment. The open letter easily got public attention, but most importantly, it got Github staff attention too. From this day on, things are getting better for the Github users. The features, requested in the dear-github letter were implemented and new improvements were introduces, like GPG signature verification, abusive users blocking, squash commits directly from Github, improved code review tools, emojis in pull requests, issues and comments and the list goes on.
Github and GitLab are both great services with their pros and cons. My advice is to try them both and find which one suits you best.