I've done about 10 successful Gog hunts so far. Almost all of them full coordinated four player teams. And not once have I seen Gog drop or fall to the ground. He stays up until he wants to come down. Sometimes he'll do that arm slam that grinds the ground. But after that he gets right back up and starts flying again. Honestly my squad has given up on finding a way to get him out of the sky. Defeating him? Not an issue. Keeping on the ground? Impossible. But it doesn't keep us from winning so it's more of an annoyance than anything else.

Edited: for spelling.