Is PIC running at 3.3v, 5V logic tolerant?
In section 27.0, "ELECTRICAL CHARACTERISTICS", under "Absolute Maximum Ratings", the third entry states that the voltage on any pin (with a few exceptions) may not be higher than VDD+0.3V. This means that the inputs on a 3V3 device are not 5V-tolerant. You will need to use a level shifter of some sort if you want to connect the GPS to the device.