Right, if you are a modest overclocker, the Gigabyte is fine. For heavy overclockers, the Gigabyte is out of the question. It has to do with the Gigabyte not having as many overclocking options as the Abit or DFI.
Then again (as I said before), heavy overclockers don't heavily base their mobo decision on a pre-installed passive northbridge cooler.
What's your definition of heavy overclocker? I plan to get at least 2.8 out of my San Diego (it's a 3700+) and I will definitely be relying on my board being able to drive the OCZ VX at at least 3.3 to get nice 7-2-2 2.0 1T timings out of it at whatever HTT I can hit, and I still chose the ABIT over the DFI precisely because of the passive cooler.
What I'm really trying to say is simply to try not to generalize; there are always exceptions, particularly in this case. As I said, I know plenty of people going for minimum 25% o/c that do everything they can to avoid active chipset cooling--it's a seriously weak link. Of course, the people I know are skilled, experienced overclockers who know it doesn't take active cooling on the chipset to achieve good overclocks.