You need to be on Android 5.1 GPE Firmware, compilation number LMY47M.M001.
Put the *.ZIP on the internal storage, then go to Settings > About Phone > System Updates and this will be applied. (This is NOT 5.1.1, it's just 5.1. No bigger System updates seem to be planned for Moto G GPE, since this device is 18 months old now).
NOTE: This wont flash in any other ROM or Firmware. Also, this wont flash on TWRP, CWM or any other custom recovery. You need to have stock AOSP recovery from GPE firmware.