Can I use GPS to synchronise two pieces of hardware?
Yes, GPS is used for exactly that purpose pretty often.

GPS receivers often emit a sharp pulse-per-second signal. Together with the current GPS or UTC time that you can query through the usual means (serial port NMEA messages), that allows for a synchronized understanding of absolute time.

For example, the Ettus USRPs (SDR devices) of all but the tiniest series can be equipped with a GPSDO, which then can be used for exactly that: Synchronizing not only sampling rate and LO frequencies, but also absolute time across multiple transceivers.