Git, on which GitLab is built, is fully distributed. Every user has a complete copy of the repository, allowing for much faster access to history, easier branching and an overall better experience.
For someone to commit to a local repository, no access has to be given in Git. At the same time, the owner of the canonical repository has the ability to restrict pushes to the repository or certain branches.
A branch in Git contains the entire history that preceeds it. It's also created or moved towards instantly and easily shared.
Git is really fast. Performing a diff, viewing history, committing and merging changes, obtaining any revision and any form of branching is much faster in Git than SVN.
A single repository in Git is typically a number of times smaller than the same repository in SVN.