I don't understand. Why the hell are you asking us instead of going directly to GBM and asking?
Give them a call and leave a message, stating your name clearly, ASAP. Tell them you can't make it.
After you call, send them an email, just to cover your bases.
At this point in time, less than 24 hours beforehand, your card may have been billed already.
This isn't really a GTAM question at all - more a personal responsibility question.