My understanding is that the GT1 class is allowed the most mods / variation from production cars. The GT2 is a step below that (allowed modifications, but not crazy ones), and the GT3 class are street cars with their original production parts that just are race prepared (ie, roll cage, fire extinguisher, removed seat / emptied out, harness, etc).
Because Matech’s Ford GT’s race in the FIA GT3 series and Doran’s Ford GT’s race in the ALMS GT2 class of their series, could it be that the GT’s are just more competitive leaving them closer to production than trying to compete in the more modified class? Just a thought. Perhaps someone with more knowledge can help out here??