I had the same tough time trying to use a GC at YYZ yesterday (gift from a family member).

Was booked YYZ-YUL-YYG on a UA award ticket (dirt status) and wanted to apply the GC to partially pay for an LMU for the first segment (credit card for the balance). Phone agent said I needed to see an airport agent due to the UA 006 ticket. Check-in agent at YYZ said I needed to go the ticketing counter.

Ticketing agent and her supervisor both reflexively said they don't accept GCs, until I showed them the above-referenced terms to the contrary. They tried various tricks to input the payment method, no dice. To their credit, they both made several calls to find someone empowered to apply the GC to my booking. Good thing I got to the airport early as I stood there for a full hour before finally getting it successfully applied.

Since I missed out on MLL access in YYZ (a minor quibble), the agent went ahead and put me in J on both segments. Great service recovery out of a frustrating situation. ^ She also handed the used GC back to me and urged me to write Customer Relations about the debacle (not sure if I will as the outcome was good).

Her supervisor also learned during that hour that GC processing should be coming to YYZ in "late July" though it truly amazed me that it didn't exist already.

Bottom line, avoid GCs at all cost and allow plenty of extra time if you're stuck with one.