The second (?) post in that thread I assume shows a standard 1/4 wave veritical with 1/4 wave radials. This is an easy to build referance antenna in the format shown, and if the lengths are correct, then its difficult to see how it is not close to 2.1dBi gain.
So if you build one of these ‘reference’ antennas, comparing the performance of other antennas to within one dBm or so is easy and quick enough.
I dont have one of those antennas so cannot measure the performance.
There is no ‘rule of thumb’ for the gain of purchased antennas.