[villagetinker]

OP, get a good flashlight, look down the drain, you should not see any gasket material. Also, as mentioned before, if your sink is located at a knee wall (not a wall that goes to the ceiling) you may be having a problem with venting of the drain. I am NOT a plumbing expert, but I believe I have seen vents that can be installed next to the sink, but this requires another hole in your counter top.

As mentioned previously, go back to your vendor and demand that this be fixed. If they refuse, stop payment on your credit card (if possible), contact Seniors against crime, and if necessary, the BBB, etc.
Hope this helps.