I have a 99' GTP that came with a non-working keyless remote at the time of purchase. I want to buy a new one that's not as worn. Does the FCC and/or GM number on the back of the remote have to be a specific number or can any grand prix remote be programmed?

The numbers on the back do matter. The correct part number transmitter for your car is 10246215, and years 97-00 from a Regal or Grand Prix are compatible. The newer model's (01-03) transmitter will not work on your car.