That happened to my mare. I honestly wouldn't call the vet for something as small as that.
Labre had a gash, really deep, exactly where yours is and I called the vet, an EMERGENCY call (which cost $$) had the her come out to look at Labre and fix her up, $450 later the vet told me that all I can do is wash it out twice a day with surgical crap, and wrap it. She still has a pretty scar from it.
The vet also told me to wrap it when I'm not home, and when I get home before I wash it out let it breathe for a 10 minutes, then wrap it up again.
EDIT: I don't think it's possible for her to get stitches, because of where the cut is located. The horse is always bending and moving that part.