What exactly happens if I leave the GSF ID blank?

The GSF ID is a (pseudo) unique identifier for your device. Every device creates one randomly when first set up and upon factory reset. Without this ID, Google Play refuses services. Therefore, if you do not provide an existing ID, Raccoon will have to create one of it's own and upload it along with the device specs of a Samsung phone.

For this reason, you are strongly advised not to mix and match. If you use your regular Google Account, also provide your real device ID. Otherwise you will find a new phone bound to your account, which you can't get rid of (Google only allows you to add devices to your account, not to remove them).