How do you check the coil in gas valve. How many ohms should it have?

The exact ohms vary from unit to unit check to see that you have a resistance and that its not grounded

You need a volt/ohm meter. You can unplug the furnace and use the ohm meter to check from one terminal one top of the furnace to the other. If the needle moves more that likely you gas valve is good. Another check would be to turn the thermostat up trying to get the furnace to light. While trying to get it to light hold the two leads on the terminals of the gas valve. Hold them there for several minutes, if after a couple of minutes you get 24 volts but the gas does not come on then the gas valve is bad.