Unable to use custom email address with GSX
Since GSX is part of the Services that Apple extends to partners that are cleared past the level of support that end users receive, you might want to reach out to your contact at Apple to verify that you can use any AppleID to access the service.

Depending on how you qualify for GSX - you might be restricted to use the AppleID you used for testing / certification or one that matches the company that qualifies to allow you to join the program.

Rather than keep making new accounts - get a hold of support or someone that has guided new users since trying to brute force your way into the system likely will have negative ramifications for your access or come back through your company to you.