With a smaller coil I think you'd get that size - around 2 grains - fairly consistently with a GPX 5000 if its right near the surface, but with not much depth at all. Much smaller and you start having issues with the GPX. As Steve said, if you are going after tiny near surface tiny gold you'd be better off with a VLF like your GM2.The newer GPX series do better with the smaller gold than the earlier units like the 3000. Other than for general coin shooting in parks or cutting through real trashy areas, I dont think you'd find the GB Pro a huge improvement over the GM2.
The problem with asking questions like this is they ignore the mineralization of the ground. Recently, I was detecting at a claim I own in the Sierras. A friend found a nice 6 gram nugget essentially on the surface with a GB Pro, and after doing so he carefully detected all around it. After he was done and told me to have at it, I took my GPX to the spot. The ground at this particular place was very noisy and mineralized. I found two half gram nuggets within 18 inches of the spot where the 6 grammer was found - one on either side. Neither was at a depth of more than 2 inches. Can the GB Pro see a half gram nugget within 2 inches of the surface? Of course - however if the ground is noisy enough, the answer is no, it will not. The strength of the Minelab PI machines is in dealing with mineralized ground that VLFs have difficulty with.
