https://wccftech.com/download-ios-12-gm/

You can now download iOS 12 GM (Gold Master) build for your iPhone, iPad or iPod touch right away. We’ll show you the routes you can take.

Apple has released iOS 12 GM for registered developers and public beta testers today. Which means that if you have a compatible device with you right now, you can go ahead and download iOS 12 GM right away.