If GM did void the warranty, you would likely have a claim against the seller (whether a dealership or private party) of the vehicle for a breach of an implied warranty. In Louisiana, this would be known as a redhibitory defect which is defined as a hidden defect in a thing which either renders the car useless or diminishes its use to such an extent that you either would not have bought the car or would have bought it, but at a lesser price. If you could prove that there was a hidden/unknown defect (i.e. a tune that voids the factory warranty) then you could either have the sale rescinded or the price reduced by the value of the warranty (depending on the circumstances). While LA law is different than the other 49 states, there are similar provisions under UCC sales law which is generally applicable in the rest of the US.
Generally, the viability of this claim would depend on the actual contract you entered into when buying the car. Usually, used car contracts contain the clause "as is, where is, and without warranties," or something of this nature. Whether or not such a waiver of warranty is effective is sometimes a complicated issue of law. As an attorney, if I were in this situation I would argue that while I did buy the car "as is," as is included the remaining GM factory warranty. The practical problem is that most people are not lawyers, and it is not cost effective to hire an attorney to bring such a claim against a seller. The moral of the story is to ask the seller whether or not the car has been modified to such a degree that the factory warranty could possibly be voided. Further, you should get a writing signed by the seller stating that the factory warranty is still in effect. If the seller is unwilling to do this, don't buy the car.
2010 Camaro 2SS/RS M6 Black on Black
22" Vossen CV1 Wheels, Roto-Fab CAI, Dynatech 1 7/8" Long Tubes w/ High Flow Cats, Magnaflow Tru-X Pipe,
Barton Short Throw Shifter, Vector Motorsports Tune, Ground Effects, MPD1 Spoiler