Did you receive it from the GM of a Radisson property? Is it valid just for that hotel or for any Radisson hotel?
Any gift certificate that is valid for all Radisson hotels states that it is subject to availability.
You say you had no luck with the 800 number. Did you speak directly to Customer Service or to just someone making your reservations? If you did talk to Customer Service, did you speak with a Supervisor or Manager as suggested?
Or are you really just wanting to rant?