Located right in heart of Clifton Hill, Boston Pizza is your best bet for good gluten free food in Niagara Falls on the Canadian side. The picture below is from the SkyWheel. Boston Pizza is also conveniently located across from the Clifton Hill stop on the red line of the tourist bus system (WeGo).
As you can see below, they had a VERY large menu. Their servers also were very knowledgeable. After the other difficult venues, this place was such a gluten free heaven that we had a few meals here.
Over a few visits, I had their oven baked wings, a salad, and their gluten free pizza twice. The pizza was the Kinnickinnick brand and extra yummy! (If you’re not familiar with this brand, first is one of the only soft gluten free crusts out there. It is also nut free, soy free, dairy free!) All was delicious and safe! Also some yummy cocktails available. The gluten eaters in my party enjoyed their gluten pizzas as well.
Be prepared to wait for a table since this is in a busy area (worth it for good gluten free food though!) and service in Canadian restaurants in general (like Europe) was much slower than in the USA, just be prepared for that almost everywhere.