The default GW does not exist. It’s a fake to satisfy the host system, emulating a L3 subnet assigned to an ethernet. It reality, the modem is connected to the PGW via a point-to-point link, and is assigned a single address only. The address you see as “GW” is most likely one of hte other PGW pool addresses, and may be in use by another modem. Or currently not in use at all. In any case, you cannot expect it to respond to pings.
Try pinging some address on the Internet which you know will respond to pings instead. My guess is that it works.