Hi,
I'm not familiar with the GSG. Do they have a firing pin safety? If yes, I would bet it is the safety plunger which lets the firing pin go past it, as the hammer strickes.
I have a Tanfoglio Match with a conversion kit. On my conversion kit I had the same things happened as mentioned. I dismantled the slide, removed firing pin and safety plunger, and found out that the notch in the plunger was machined slightly off. I noticed slight peening marks on one side of the notch in the plunger where the firing pin struck. That means, the firing pin struck the safety plunger first, bounce off it and just barely dent the rim on the cartridge. The interference was just enough to slow the firing pin down on the way to the cartridge. I fixed it with a few strokes with a needle file. Never had a problem since. If you can't do it your self, bring it to a gunsmith and have him check it out.