After reverting back to their base form, they absorbed the god ki, allowing themselves to mix their Super Saiyan ability to become SSGSS. Prior to not having any godly ki, they required 5 other saiyans to become SSG. Mid way through battle against Beerus, you'll see Goku become SSG On his own. Goku and Vegeta never lost SSG. Saying that is like saying Goku and Vegeta lost Super Saiyan 1, and 2 because they don't transform into it anymore. SSGSS is stronger.