I'm signed up for GSJ and have ordered from him in the past ... HOWEVER ...
Without logging in, I filled everything out on GSJ check-out ... the first checkout page has a link to PayPal but I DID NOT click on it ... I filled all information including coupon code ... and clicked "Continue" button.
Then I filled out the second GSJ checkout page with my BF member name in the "Special Instructions" ... THEN and ONLY THEN, when there were no more Continue checkout options ... I clicked on the PayPal link button.
Everything went smoothly this way and I received a GSJ confirmation as well as a PayPal confirmation.
* * SPECIFIC TO MY BROWSER SETUP: I'm running Firefox in Linux Mint and Firefox is set up with "Multi-Account Containers". I had to restart Firefox in troubleshooting mode so that all browser add-ons were disabled. In this way, GSJ and PayPal were able to "handshake" and complete the transaction ... without being blocked by my use of "Multi-Account Containers".