I think the consensus will be drain and fill (and filter). No flush. I'm not a transmission mechanic - only repeating what I've read - but it seems reasonable. And that's what I've always done on all my vehicles.So, fluid only? Fluid and filter? Prayers?
It's not clear what model year you have so I'm not sure what transmission is in your car, but the later 6T70's don't have the bad rep that the earlier 4T65E's did. So there's a bit less concern if you have the 6-speed 6T70.
I believe the gasket is re-usable, so, unless it's damaged, you want to keep it rather than use the one that comes in the filter package.