I haven't worked on the GP100, but found the light firing pin strike problem on Blackhawks that had trigger work to lighten pulls and balance the trigger spring to reduce backlash. The transfer bar wouldn't move fully upward because there was no backlash (that a factory pull commonly has, to get the bar high enough to adequately transfer force to the firing pin). Installing a trigger stop could also create the problem.
The way I often fixed the problem was to remove the transfer bar, heat it just short of red hot, then lengthen it by peening. After peening, it may need to be ground/polished smooth. Casenit can be used to harden the end that's struck by the hammer, preventing deformation.
(In older transfer-bar system guns, the transfer bar might be deformed by hammer strikes to the point that such light strikes can occur in unmodified handguns. Replacing the transfer bar usually takes care of the problem.)