The particular baggage check-in was processed by Air Canada without
providing any excess baggage ticket or coupon to the passengers who
have interline connection with another air carrier.
Has this actually ever happened to anyone with gate-checks and multi-airline itineraries? I've never heard of such a thing where a carrier would provide "proof of gate-checked bag" coupons to show to gate agents on connecting flights with other carriers.
Never allow forced carry-on gate checked luggage to be tagged to final destination on another airline.
While this is an obvious thing to do, I have seen gate agents become rude and hostile to passengers if they don't comply and gate check the bag anyway.
This is clearly the responsibility of the airline that gate-checked the luggage.
It is entirely caused by Air Canada
I do agree with this and have sent them an email about what happened. The compensation will probably be one of those "15% off base fare" codes so I'm not expecting much when they eventually reply.