Oh, I have definately stripped an action screw, on a MDL 700 in 1976. I had tried to use a broken bladed screwdriver to remove it and smeared the notch and couldn't get any driver to bite after that. By all means hope that that is it. It took gunsmithing to correct, but the screws should be softer than the reciever for just that reason. If the thread were damaged, I think you would be able to see the damage.
I hope your GS is able to fix it simply. Good luck.
Doh! Now I see you have it fixed. All is good then. Amen!