2 hours, well depends how you see it, HK airport is quite efficient, I find it more than enough. That's how long we had last time, made it comfortably through it and had time to visit a lounge, we spent less than 30mins from the time we get off the plane and back to the departure floor. But it assumes your first flight arrive on-time though.

I have no experience with gate-checking a bag, other FTs may help on this point.

I wonder if it is possible to check-in a bag at CX transit counters on the arrival floor, before you head to the departure floor through the transit area?