I have the same type of car charger that has the two USB ports, and you plug your USB cable into it to charge your phone. I used it with my Droid X, both directly and through the car dock. I'm not sure if it came with the car dock or if I bought it separately.
But anyway, no matter what cable I use, including the one that came with the G'Nex, it won't charge the phone. I checked the same cable(s) with my Droid X in the car and also with the G'Nex on my computer and the cable works. But if I pull the USB cable out about halfway on the car charger side, there's a small range that will make the G'Nex charge. If that doesn't make sense, I can take a picture.
I found that pretty weird. Is it typical for this phone? Should I go ahead with the Griffin charger and be able to use it with the Samsung cable that it came with (or other micro USB cables)?