without load, the multimeter will show you the battery voltage, so no reason to messure it bill. Now, under load, you will not be able to see something because drop voltage depends on resistance. The bigger the resistance the lower the drop. If you really want to see if GG has small drop voltage you have to compare it with another mod. To my opinion there is no reason to do that too because GG is steel and have brass contacts. That means that you have to compare it with a same style e-cig. If for example you compare it with an all brass e-cig then GG will have more drop. If you compare it with an all Steel e-cig then GG will have less drop.
Those comparisons where good only when people were trying to find the best materials to make an e-cig. In our days the best combination is what GG has, that is a strong construction made from SS and brass contacts for the best contactivity.
The conclusion is that you have already the best and there is no need to test anything