It is actually indicated for gapeworms as they are a form of roundworm. Good news, that. It doesn't kill tapeworms because of its method. (For that you'd need a different wormer.) It kills most all other common poultry internal parasites (not pathogens).
If you use ivermectin, I would highly recommend a first worming with a lesser wormer like Wazine (piperazine 17%) then following up with ivermectin in 2 weeks. ivermectin is SO broad-spectrum that it kill nearly every worm in the bird. If the bird has an unknown heavy infestation, the dying/dead worms that are passing out of the body can clog the digestive tract and/or cause a foreign-protein reaction. So the conservative protocol in flocks with an unknown worming history (or if you see them passing worms) is to use Wazine first, then something more broad spectrum (as wazine usually requires multiple applications anyway by design).
(Thereafter, I worm twice yearly with it unless I need it more often - without the wazine pre-treatment).
You can do it the other way, ivermectin first (particularly because gapeworm is a harmful parasite that acts more quickly than other worms). But just know the risks.
It's one of those double-edged sword situations - either choice would be acceptable.
By the way, the best place to drop it is inbetween the shoulderblades. Make good and sure that you separate the feathers with the fingers of one hand so that you get a clear spot with no feathers. A good place for a drop to soak in. Drop-on soaks in amazingly fast and you'll lose a drop on a feather without getting its effect. So find skin, drop there. The back of the neck also offers a lot of bare spots.
Wear gloves. I'm probably worm-free from many misplaced drops of ivermectin! >.>
