The info that is needed to do this conversion is missing form you post. The main concern here is the size of your BB - on the bike - not the bearing that fits into it. BB30 means that the thickness of the shaft running from one side to the other is 30mm also known as OD - outer diameter. A GXP crankset has a 24mm shaft on the drive side and then a step down to a 22mm on the Non drive side. This keep the crank from sliding side to side. If you have a `SRAM GXP, outboard bearing bottom bracket` which would leave me to believe that the bottom racket on the bike is threaded and in that case - NO - to my knowledge there is no adapter. New bikes have non threaded BB that are bigger - BB30 has OD of 40mm and PF30 has OD 46mm - these allow for the bigger bearings to fit into the bike frame. In new frames with BB30 and PF30 bottom brackets you can fit spacers but unfortunately not the other way around - as far as I know. BB30 and PF30 are also narrower than GXP - by about 40mm if I remember correctly - so even if you could get the bearings to a bigger size to accept the bigger shaft - stop the jokes, I know what that sounded like - the BB30 crankset would still be to narrow and not fit. Hope that helps.
