"As a Star Alliance Gold member, you are invited to relax in the Air Canada Maple Leaf Lounge as you wait for your Air Canada, Air Canada Express, Air Canada Rouge or Air Canada codeshare flight, or for a flight with one of our Star Alliance partners, in any class of service." https://www.aircanada.com/ca/en/aco/...f-lounges.html

I believe UA Plat = *A Gold, so you appear to be eligible, but since your boarding pass will not show your status, you would need to show your card.