Oil leaking somewhere?
ive never had any problems with oil leaking from the waterpump... but theirs always something 
The timing cover I would have to say is unlikely unless you lost that amount of oil over a long period of time. Usually a timing cover will leave an easily noticable spot on the ground.
Id take your car to the wash with some engine degreaser, give your engine a good cleaning all around while being careful of the electronics of course. The just drive around for a week, jack up the car and start searching for any signs of your leak.
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Take it to a shop and ask for the dye. They have an additive they put in your oil and drive it around for a few min. Does NO damage to the engine, but they shine a blacklight under the car and any fresh oil that has that additive in it will glow flourescent...will help locate that leak...
your cover will look like the one on the left. the seal is the middle hole. You can pick up a timing chain gasket/seal set from autozone for like $10. it includes all 3 seals, 2 waterpump gaskets, and the cover gasket. or if you want to stay GM. the opti seal itself is GM#12552428
what seal behind the retaining plate? you talking about an LT1?



