I had this same problem. I was parked and all of a sudden my oil light flashed and then stayed on. Checked under the car, and all my oil left the car! I took it to the shop and they immediately said "Oil pan gasket.." and tried to charge me about $250. However, looking into it more, my friend Josh and I discovered that it was NOT the gasket, but the rubber seal around the gasket that had slipped out - this is a reacall or a TSB (I forgot which) for our early model g3's. Otherwise, just jack the car up (or put it on a lift, thats better) and on the passenger side towards the front (where the oil pan and the gasket is located) and slip it back in there and refill with oil!
I thought my pan gasket was leaking too, but after further investigation I found my leak to be the oil pressure sensor. The oil has dripped down and leaked all over the pan making it look like the pan. and like firespirit said it could be that rubber thing. So just make sure it's the pan gasket and not something stupid and easy to fix. I have yet to fix my pressure sensor but only because I leak about a 1/2 quart between oil changes.