How to repair an GFCI outlet with a loose ground (green) screw?
I managed to fix it!

It seems there is also a little hole for Push-In ground wire and meant to be tighten with screw. The push-in hole gave me an opportunity. I first line up the inside and outside holes by gently shaking the receptacle. And then I insert a needle, in-lieu of the ground wire, through the push-in hole to apply pressure to the small holed plate from the inside. Finally, I screw in the screw. Good as new!