That's a pretty good sized leak you have there. I agree with a possible evaporator issue. If yours has rear air then you also have to look at the rear a/c components as well. The biggest cause of GM a/c leaks I see at my shop is the compressor. They will leak out of the front seal, which can be hard to detect without removing the clutch assembly. They also are prone to leak out of the center of the compressor where the two halves connect. Be looking for traces of refrigerant oil on hoses and system components. Don't overlook the check ports on both high and low sides. They are a common leak point. Good luck. Freon leaks can be a pain to find sometimes.