I'd like Garp to react to the genocide game on GV (like when Kuzan reacted to Akainu blowing up a ship full of civilians in Ohara) but Oda will likely have him only focus on Roger/Xebec because he's a cowardly writer
I think Garp will command some Marines to protect the Citizens, without hurting the Celestial Dragons, so they save lives, but, don't commit the crime of attacking CDs.
I think a lot of Marines will be Domi Reversi'd.
IMO, the people who survived God Valley were:
-Plot characters we know of who are important.
-The people Kuma saved
I think almost everyone else died, including most Marines. We haven't seen a single named Marine who's not Garp or Dragon here, as far as I know.
If Garp watched a bunch of Marines help hunt human slaves, then turn into Demons, and then all die... I can see why he doesn't like talking about God Valley.
It's also possible Garp fought the Celestial Dragons here, and rebelled, but it was covered up along with the rest of the incident.
The only Celestial Dragon characters we've seen here that I know are present in the modern era are Gunko, Garling, and Sommers (Shanks and Shamock, too).
It's entirely possible Garp fought/killed God's Knights, but they died, and no one found out, because every Celestial Dragon besides the five I mentioned died here. Hence, "God Valley."
I don't think there were many witnesses at the end of God Valley.