After some trials I find this GPSDO is not so good after all.
The 10MHz output is stable at about 9999999.9991 Hz, it appears
locked by the GPS but at the wrong frequency due to some defect
in the control loop. It is apparently a known problem and
appears on all the variations of BG7TBL's GPSDO.
Another problem is that you cannot send RS232 commands to the GPS
chip - its serial input is taken up by the on board controller.
So I can't recommend this one. It's a pity because it's a
neat low cost unit.
Now I'm about to buy one of these instead