Depending on the resolution of your ohm meter, a glow plug should read NEAR 0 ohms. Zero ohms is a dead short, and obviously the glow plug is not a dead short, but it is darn close. on average a glow plug will draw close to 20 amps of current at near 12 volts. Keep in mind the resistance of the plug will change dynamically as it heats up, just like any heating element. So lets explore Ohm's law. E=IR where E is Electromotive force measured in Volts. I stands for current which is measure in amperes (amps) and R is the resistance measured in ohms. So with simple 9th grade algebra we can plug in the values we know and solve for the values we don't know or want to do a "sanity check" on.
So the voltage we will call 12 volts (that is the E) the current we know from manufactures specs at 20 Amperes (that is the I) and the resistance of the glow plug we will solve for in the following equation
12=20R so lets divide through both sided of the equation by 20 we come up with 12 divided by 20 =R so 12 divided by 20 is .6 so R=.6 ohms
So your glow plug is about .6 ohms or just over 1/2 of an ohm. Again depending on the resolution of your ohm meter, it will measure near zero.
I hope this helps.