gc stands for garbage collection.
While doing some operations like git reset or git rebase, the history can be altered and Git accumulates various types of garbage like innaccessible commits, that can still be checked out, cherry-picked etc...
git gc will clean detached commit and also compress stored Git objects, freeing disk space.
git gc is run automatically on commands such as git pull, git merge, git rebase and git commit but running it manually is needed for frequently updated multi-users repos.
A similar command is git prune. git gc is a parent of git prune.