GSX Level 2 is a payware expansion for GSX so, if you bought it, you surely must have a GSX Level 2 Serial Number (which is different from a GSX Serial Number) in your email, and of course if you lost it, you can check the FAQ page to retrieve it.
If you never had a GSX Level 2 purchase receipt, it means you haven't bought it, so it will work in Trial until you do.