I don't know for sure. You are correct that the snail has to be infected to pass it on. I don't know enough about the life cycle of the gapeworm to know if it is even possible the parent snails could infect their offspring. But I'll give you my opinion.
Earthworms, slugs, and snails can be carriers for gapeworm. Grasshoppers, grubs, and other critters can possibly be carriers for other parasites. But they also have to be infected before they can infect your birds. I don't try to stop my chickens form digging up or catching these critters when they are out foraging. If I find grubs when working in the garden I might save them and feed them to the chickens.
My situation is a bit different from yours in that the critters I'm talking about are in their environment anyway and they are going to be exposed to them whether I feed them grubs or not. Personally I'd take the chance, figuring the odds are pretty low and I like to take advantage of free opportunities. But if you are uncomfortable, don't. What do you actually gain by feeding them those snails? Is that gain worth the risk, however small it may be?
