How can I put the GPS module to sleep via the command line?

The GPS module can be put to sleep by sending a UBX command to the GPS module.

To put it to sleep;

echo -e -n "\xB5\x62\x06\x57\x08\x00\x01\x00\x00\x00\x50\x4F\x54\x53\xAC\x85" > /dev/serial0

To wake it up;

echo -e -n "\xB5\x62\x06\x57\x08\x00\x01\x00\x00\x00\x20\x4E\x55\x52\x7B\xC3" > /dev/serial0

The UBX protocol is documented here.