What is the gain for a particular CB antenna - What...

Gain can be measured either against a 1/4-wave antenna or 1/2-wave. For most antennas, the old industry standard was to compare antenna gain to an isotropic antenna, which is a theoretical antenna that has the gain of a 1/2-wave antenna, but all emissions emanate from a pinpoint (that's why it's only theoretical). An isotropic antenna can be loosely defined as a single half-wave element. CB radio frequencies center around 27.2 MHz. By today's standards, that's a low frequency, even though back when it was named, it was called the "HF" or High Frequency band. The lower the frequency, the LONGER the wavelength. The longer the wavelength, the longer the antenna needs to be to have any gain. There is a commonly used formula to calculate the starting wire length for a 1/2 wavelength wire antenna:
Length = 468/Freq
Length is in feet, and Freq is in MHz. For CB, we solve this as follows: length = 468/27.2 = 17 feet, 2-1/2 inches. So, we now know that an antenna of about 17 feet will have the same gain (zero gain) when compared to a 1/2-wavelength antenna.
Over the years, antenna manufacturers played a specification game with each one trying to show their antennas were better, and eventually someone started comparing to a 1/4-wave whip instead of the isotropic. Since a 1/4-wave has only half the gain of an isotropic antenna, they were able to advertise twice the gain. However, the antennas themselves didn't get better - only the advertised gain number changed. It's important when you see a gain specification to find out what the standard reference antenna was.
In CB, the standard reference antenna is a 102" whip. Since all mobile antennas (except the 102" whip, of course) are shorter than 1/4-wave, performance will NOT be as good as the 102" whip. So, no one in the industry has ever published gain specs for mobile CB antennas - since they would ALL look bad!
Even heavily advertised branded antennas compare their antennas to other comparable antennas. You may notice that they are not compared to a 102" whip, and it's easy to understand why. These expensive antennas are not as good as the simple, inexpensive 102" quarter-wave whip antennas.
We suggest that you purchase the largest CB antenna that is practical for your vehicle. You should consider whether your vehicle needs to get in and out of a garage, and where the antenna will mount in order to make a good decision. All claims aside, in mobile CB antennas, there's just no compromise for length!