Powering the GPS shield

I have the MKR GPS Shield. On the package it says 5 V input voltage and 3.3V operating voltage. On this page, however, it says input voltage is 3.3 V.
I want to involve it in a system with a non Arduino board, which runs on a 3 V coin cell battery. With its own cell battery I guess the 3.0 V is sufficient for the shield to run. The logic level is assumably 3.0 V if the power source is 3.0 in the microcontroller board as well as in the GPS shield.
I guess the only connections I need are TX, RX and GND, while both units run on their own coin batteries. Or is there something to gain in connecting also the 3V? The GPS shield will mostly be asleep.