I've never had gapeworm in my flock. You can contact the UMO county agricultural cooperative extension. They have someone who specializes in poultry who might be able to advise you. IMO, the more you read (and I read A LOT), the more you'll hear about all the things that COULD affect your flock, to the point where you'd be tempted to never let them experience the thrill of being chickens! My approach is to let them eat what they want, not worry about it, and if I SEE a problem, then... I'll cull or treat, depending on what the problem is and how prevalent it is in my flock.
Welcome to BYC, and specifically to the Maine thread.