This ideal is purely my own, I do not claim that it is THE skeletal and muscular physiology of a Gryphon. Please Do NOT Steal! You may use it for your own reference, but do not claim it as your own....

The red dot is the center point of gravity. I also provided a little graph of the leg and wing movement; colour-coded.

If you want a rundown of flight, check out:
Pegasus Physiology:_Skeleton
Pegasus Physiology:_Muscle Structure

I believe the flight pattern and structure wouldn't be too different, so you can interepret the pegasus info and think of a gryphon.

I didn't feel like doing a muscle structure image, but you'll get the idea of what it almost looks like if you look at the pegasus muscle structure.

As I said before, this one is completely of what I think. References of bird and lion skeletons were used, but the fusion of the two is how I see it can be done. The combining of the breast bone and rib cage was a pain.
Junk DX