A resistor can be used to drop voltage but they arent nearly as efficient as something like a Zener diode like TNT mentioned. Resistors can put off a lot of heat and waste a lot of energy. If you're not an electronics person though then we just use BECs (Battery Elemination Circuits) that you can...
First thing I would do is hook up the X-Bus to the ESC and use a multimeter to check the output voltage on the black and red wires that plug in to the GPS sensor. I'm assuming since its made for Spektrum sensors specifically it probably would output the proper voltage.
Worse case scenario you...