The Wi-Fi antennas built into the Balance One and most of the Pepwave WAPs, they have a fixed gain based on the devices design and it is unlikely that you’ll be able to get this information, the radiation patterns are going to be most likely the best you can do on these.
With the devices that can have an external antenna attached, then the gain is dependant on your choice of antenna. We normally discard the shipped antennas (such as those that come with the MAX series) and use purpose chosen antennas. The antennas that are shipped are normally only used to pre-prevision the the unit.
Happy to Help,