You have it exactly backwards. GS's are not a proffer. It is assumed that if you put in a request to fly a GS you actually want to fly it. There are some exemptions and reasons you might not be available but they are not proffers.
WS's on the other hand are proffers if its for the same day or less then 12 hours from initial notification attempt by the company.
WS's beyond same or next day coverage are not proffers and you must fly them. The company has no notification responsibility in this situation. If you have a WS in you are required to check yourself to see if it was awarded.