You're correct, gapes infect the trachea. Published info using albendazole for gapes- most mammals (see Treatments in this link)I haven't seen them either, but I'm pretty sure they don't live in they esophagus. I'm also not sure if there is any published info on treating gapes with albendazole. I have no doubt that it would be effective, but I don't know what the effective dose would be.
http://en.wikipedia.org/wiki/Mammomonogamus