There are literally dozens of gaskets on your engine, and each one takes a different amount of time to replace so the cost can vary by a huge amount. To answer your question, we need to know exactly which gasket(s) you are referring to....
On your engine, there is a GM technical service bulletin (#10-06-01-008A) describing a change to the sealing method for the oil pan if a leak should develop. The original design oil pan gasket is no longer used, when servicing the oil pan for leaks they now recommend using a different type of gasket and special GM RTV silicone adhesive material.
Parts and labor prices do vary from one shop to another, and from one area of the country to another. I can give you a general price for this repair, but the cost may vary somewhat depending on who does the repair in your area.
You will need a new oil pan gasket (GM #12587964; $55.53)
You will need a tube of the special silicone gasket material (around $20).
You will need new oil and filter (around $25)
The labor time required is 1.5 hours. At a median national labor rate of $75/hr, this would be about $112 in labor.
So, this repair typically should cost approximately $212 plus tax.