Is the light just on steady? Does it say 'Check Tire Pressure'? Or is it flashing and say 'Sevice Tire Monitor System'?
If it is just steady on and saying a tire is low you may have to reprogram the tire pressures in the remote control door lock receiver, as your new tires may not be rated for the 80psi in the rear that GM has it set at. Anyone with a Tech II can program that. Dealers may be hesitant as this sorta violates a federal law.
If the light is flashing and it says 'Service Tire Monitor' something is wrong and either a sensor was broken or not learned right. If that's the case you can try to relearn your tires yourself using this procedure:
1. Turn the key to the run position
2. Press and hold both the lock and unlock on your key fob. The horn should honk and 'Tire Learning' should pop up on the dash.
3. Go to the left front tire and start deflating the tire until the horn honks. Proceed to the RF tire and do the same thing, followed by the RR then the LR (fyi the turn signal will light up for what ever tire your on)
4. After the left rear tire it should honk twice. The light will probably be on as your tires are all low now. Air up the tires to GM's specs and drive it. The light should shut off in a block.