There's quite a few around if you google. They seem to vary from being entirely without sugar, to being half sugar half sweetener.
Having made jams and preserves for years, I'd probably go for one that does have some sugar in otherwise you won't get a good set and will end up with gloopy grape juice.
Just a word of caution: Grape Jelly is always going to be fairly high carb whether you add sugar or not, as it's made from grape juice and the fruit has a high natural sugar content.
An idea, and this is only an idea - it may not work... would be to experiment with adding gelatin to the grape juice. Not enough to make it a proper (british) jelly, but thick enough to spread ?