Raspberry Pi: Voltage Converters and IO-Pins
As pointed out before, build a 6V power supply and connect transistors (possible FETs) to the GPIO pins to drive the display. If you have trouble, check:


is it safe to drive a MOSFET from an output pin of a microcontroller?
Microcontroller Interfacing –  Part 9: FET Transistor Switching
When is a MOSFET more appropriate as a switch than a BJT?
Basic Transistor Drivers for Micro-Controllers