Note: This feature was introduced in GitLab 8.4

The housekeeping function runs git gc (man page) on the current project Git repository.

git gc runs a number of housekeeping tasks, such as compressing file revisions (to reduce disk space and increase performance) and removing unreachable objects which may have been created from prior invocations of git add.

You can find this option under your [Project] > Settings.

Housekeeping settings