[peff]

`git gc` will not remove objects mentioned in the reflog. The reflog entry for a commit that is not referenced from any refs (e.g., because you deleted the branch it was on) will last for `gc.reflogExpireUnreachable` time units. By default this is 30 days.

After that, then the object is subject to pruning by `git gc`, and may go immediately, or up to 14 days later, depending on the packing. That 14-day grace period is there for objects that never got referenced in the first place (so a tree that you are building to make a commit, for example, would not go away before you run `git commit`).