Talk to a lawyer. Garretson's may not have a legal responsibility to check the gun but they could be held liable for the contingent costs associated with your spoiled trip. You could recover your costs and be paid for your trouble.
Luckily I brought two shotguns with me on the trip, so I was still allowed to go on up and hunt with the other one.
What I think is really needed is for the ATF to grant access to some sort of on-line database for gun dealers. The Canadian Customs guys have access, why not a federally licensed firearms dealer?