This HAT is pretty simple but its worth going thru what pins are used for what!
The Ultimate GPS uses the following GPIO pins: TXD, RXD, #4 and EEDATA/EECLK
The Raspberry Pi has only one serial port, and you do need serial to chat to a GPS so we will take over the RXD and TXD pins.
GPS's can output a 'pulse per second' for synchronizing the time. We have a breakout for this and a closed jumper that connects it to GPIO#4 - if you do not need PPS and want to use #4 for something else, just cut the little connection on the PPS PCB line.