Why didn't my turkey stock gelatinize?
My best guess is that your stock is very weak.  Two gallons of water to just the bones from one turkey will not be a strong stock.

It may well well have gelatin in it, but very thin.

I make stock for a single turkey with the wing tips (not the 'drumstick' part), the back, the neck--everything but the breast and leg/thighs in with about 1 gallon of water to start, which reduces to several quarts over the course of three-four hours.

This produces a very shaky jelly--much thinner than the one in a jar of Smuckers for example.