If gedit is already open and you are using gedit as your git core.editor then it will return immediately instead of forking. This will give git an empty commit message error.

Simple Solution

Add a flag to fork gedit for situations like this.

Redhat has a similar report: https://bugzilla.redhat.com/show_bug.cgi?id=648111
Stackoverflow has one too: http://stackoverflow.com/questions/60278/git-commit-text-file/4658092#4658092