GPS "red X" means that it cannot talk to GPS at all. First, understand that plugging Revo in to USB does not power the GPS. You must have flight battery plugged in to ESC/BEC to make 5V for Revo for power to go to MainPort / FlexiPort.
FYI: GPS "red block" (not red X) means "good but GPS has not got position lock yet". Yellow means it has a position lock but not good enough to fly. Green means GPS ready to fly.
If not simple power issue, it could be wrong cable. For serial function, the cable must be made with Tx and Rx "crossed". "FC (Flight Controller = Revo) Tx must connect to GPS Rx" and "FC Rx must connect to GPS Tx". If this is your problem, you can use #11 model knife blade to very carefully only slightly lift the tiny tiny plastic flap on the cable for these pins. Slide Tx and Rx pins out and slide them back in in reverse order, Rx and Tx.
I saw your other post, but I had to use Google to translate. If MainPort is not working, look in the FC MainPort connector to make sure that the pins are straight. #11 model blade will get the straightening started, but then you need a super thin needle nose pilers or maybe hemostat to squeeze the pin straight. Be very gentle if what you use has teeth to make sure the teeth don't make marks in the pin.