I'm going to guess that you are using a analog multimeter - one that has a swinging needle to indicate the reading (not a digital meter, which usually reads out on a LCD).

And I will assume you have the meter set to read ohms on one of the scales (should probably be the lowest scale).

So, when the meter swings all the way over, that is zero ohms (or close to it). The meter swinging all the way across the scale should indicate continuity through the plug. Good.

If the meter does not move, that is infinity. No connection. Bad glow plug.

It looks like you have narrowed it down to GP3 properly. Unfortunately this one is not easy to get to. I think you'll have to remove the intake manifold to replace it.

Ken300D