I never tried Class B yet, but I found a mention from Brocaar in another topic regarding GPS …
Please note that in order to use Class-B, you need to have a gateway with GNSS / GPS time synchronization.
Plese also check that the configuration should be like this running on ttyS0 in some cases of GWs.
{
...
"gateway_conf": {
...
"fake_gps": false,
"gps": true,
"gps_tty_path": "/dev/serial0"
}
}