They aren't going to put you in GA, if you have a GC ticket!!
But the other way round can, and does, happen.
My brother had GA for U2 at Manchester. He and his wife, ended up being pushed into GC. And they were right at the front of GC! So it can happen.
But, I dunno about what time to que. I'm not in GC this time, so...

But I'm getting there a few hours early, even with GA. Not just because I wanna get as close, in there as possible. But because we don't know our way around Twickenham. And by the time we've got the train there, and blah blah...that's when abouts it'll be. Might as well start queing!
I just know that, if I had GC...I'd get there as soon as I could. I'd wanna be RIGHT at the stage. lol. Have fun.