https://wccftech.com/download-ios-12-gm-without-developer-account/

Want to download iOS 12 GM without a developer account on your iPhone, iPad iPod? Here’s how you can do it right away.

As you might have figured by now: iOS 12 GM is meant for developers and public beta testers only. If you aren’t in either of the leagues mentioned before, then you are out of luck downloading iOS 12 GM.