First, eBay's software has some features that are either obsolete or don't work. So, just because you might have checked something upon registration, doesn't mean that it still works or will work.
That said, GSP is an option that can be selected with every listing. You can edit a listing, and uncheck GSP if you don't want to offer it.
How did it get checked in the first place? If you are using the Quick Listing Tool, a lot of options are set by default ... also, even if you are using the Advanced Listing Tool, the system tends to remember your last preferences, so if one listing had gotten checked, then subsequent listings would also have been checked.
