Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Disconnect the three wires on the gas valve solenoid taking care to note their positions and measure the resistance across one of the outer terminals and the middle terminal on the solenoid.
Repeat the measurement using the other outer terminal and the middle terminal on the solenoid.
Both solenoid resistances should be approximately 105 ohms.
Reconnect wires.
Repeat the measurement using the other outer terminal and the middle terminal on the solenoid.
Both solenoid resistances should be approximately 105 ohms.
Reconnect wires.
Isolate as per TOPs P4
and refit as necessary