As far as I know (not much) 6" half lug GP-100s have been made on and off and the last batch was made only about a year ago. They had them at Davidson's.
I was just recently, researching the GP160 half lug revolver and had found that this was a Limited Edition, last produced sometime in 2008 along with the KGP160 stainless version of this revolver. In fact, I've got a line on an NIB GP160 that I'm intending to buy!