Gray market dealers do sell authentic watches, but they often remove serial numbers so the watch can't be traced back to its source. If yours has its serial number in tact this a sign that you probably bought from a "better" gray market dealer. There is no such thing as a good gray market dealer, so when I say "better" I just mean that they had the decency to leave the serial number in tact.
Your watch should fall within the COSC spec of -4 to +6 seconds per day. -16 seconds per day is well out of this spec and your watch requires service. Unfortunately, because you bought from a gray market dealer, you have to go through THEIR warranty proceess. This means they can send the watch to any old jeweler they want to try to fix it. If you're lucky he'll fix it right. If not, you're in for lots of headaches sending it back until it is fixed properly.
I'm sorry for being so blunt, but had you bought from an authorized retailer you probably would have paid about the same price and been covered under Omega's FACTORY warranty. Your situation reinforces my belief in avoiding gray market dealers like the plague.