From the back at the gasket. I've tried to re-seat it three times now, and it's a brand new gasket. Last time was with a poop-load of RTV, and it still didn't work.

Here's a thought: When I first replaced the gasket, I completely unbolted the oil pan, but had not taken the tranny shield off (M6). So the oil pan was hanging, biased towards the front, and being supported only by that tranny shied at the back mating surface of the pan. Could the weight of the pan only (it was drained) have been enough to tweak that mating surface to the point where it wouldn't seal correctly???

That's the only thing I can think of at this point. The gasket was a brand new GM pan gasket, and it was NOT leaking from the back before I put the new gasket in.