Thats kind of a hard question to answer, because different concentrates are concentrated at different levels.
one might be 1.035 gravity, another might be 1.045 because its been concentrated more.
I will say that in the recipes section there is a frozen juice concentrate recipe posted. It uses 3.5 cans of frozen grape juice (i.e. Welches, Tree Top, etc) per gallon, and a bit of acid blend to balance the flavor.
Otherwise you'd have to get juice contentrate, and determine the gravity.