You said the voltages on the gas valve were correct - but what have you got? I can't find a figure in the MI. Worth a call to GW. Called GW recently about a problem which could have had several causes. They wouldn't admit a "common fault" but directed me to one of the half dozen which was a bit obscure - and they were right.
Please tell us - useful to know these things!
(NB be careful when measuring voltages like that. Unless it's pure smooth DC your meter might give a different reading from someone else's. If you put it on a low range AC you'll see if there's any "ripple" on the DC which can give the different reading)
You also need to check the resistance of the gas valve coils. If they're too low, it can drag down the voltage coming off the pcb,and make you think the pcb is faulty. Again, GW ought to know, though you may have the old valve to compare it with?? If you suspect that, it's worth measuring the voltage coming off the pcb with no gas valve connected, which would normally be a bit higher.
That should give you a 90% certain diagnosis on whether its the gas valve.
If you think it's sticking a bit, tap it with the handle of a hard plastic screwdriver and see if the pressure alters much - you'd expect a jitter on a good valve, but not a jump of a couple of mbar say.
To decide if its the pcb you might have to check everything connected to it, and you get the same sort of slight uncertainties with readings, so 100% confidence is evasive.