Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect gas valve plug from PCB (rightmost plug)

Measurement:

Measure the resistance across the brown wire to the blue wire next to it at the disconnected plug.
Resistance should be approximately 4.3k - 5.3kΩ.
Note:
Two types of gas valve lead may be fitted, refer to the image that corresponds to what is fitted on the appliance you are working on.