Which Oncore?

5.3 says GPVTG is on the GT, SL and M12, not the UT and M12T.

It looks like on 5.153 you need to send $PMOTG,VTG,0001 (with optional checksum) to for example enable 1 per second GPVTG messages. If you want I'll build you a custom gpsapp to try which sends this and you can see if it works for you?