I have never seen a grape juice or jam or sparkling juice product that had gluten ingredients in it. I'm not saying it's not possible, but if a gluten ingredient isn't listed on the bottle, I *personally* wouldn't worry about it one bit.
I drink Kedem all the time with no problems, as do Celiac Jews that I know. Especially at Passover.
IB
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Gluten-Free Bay
http://glutenfreebay.blogspot.com