Firstly is there a way of storing a private key with a script and have Crypt:GPG decrypt a file using it?

There is no need to store the private key. You use it to "generate" (encrypt or sign) your "license key". For decrypting/verifying the public key is used, which you can store more or less "open" in your program, because it is, so to say, "public".

(Of course, not too open, otherwise it would be easy to defeat)