Currently, the SMALLEST GPS transmitters/trackers available are about half the size of a blackberry cell phone, and just as thick, and require recharges quite often. RFID chips are a totally different animal, for example commonly embedded in hotel room keys and corporate ID badges, and essentially never require a recharge.
Beyond that, a programmable RFID chip DOES help identify the legal owner of a firearm, if said owner should choose to reprogram their chip. Frankly, if the gun is stolen, it'll be held as evidence, and even if the true owner IS proven, they'll likely never get the gun back anyway.
A GPS system would be essentially pointless. Again, the batteries and size would be the biggest initial drawback, and beyond that, if I KNOW my gun has a GPS transmitter in it, I can jam or disable the signal quite easily. Further still, Who will monitor the flow of MILLIONS of firearms moving around the country at all times? What servers would receive and store said tracking flow data? How would this PREVENT crime? It'd make more sense to have metal detectors in liquor stores, that way the clerk would at least have a warning that someone coming in might be packing. The only thing the GPS unit would tell you is where the perp dropped the gun after he already shot the clerk.