Technically, it's not NECESSARY, but....if you were to input the wrong value (too low) you could have a situation where the AP may exceed local regulatory power levels (Illegal and possibly subject to fines).
As for finding the actual gain (dBi) value for an antenna, you will always be able to find this. Besides the horizontal and vertical bandwidths, it's one of the most important characteristics of any wifi antenna...and as such, it will always be stated in the description of each antenna.
So once you have this value, input it into the antenna gain field.
But now we're back to your original comment - using a combination of different antenna types - with different gain values. So which gain value do you use now? To be safe, you would use the larger gain value.
But, as I mentioned before, you create other issues by having antennas with different radiation properties and gain values.
So bottom line, use the same antenna types for each radio (perfectly fine to use a DIFFERENT antenna for the other radio though) and find and input the antenna gain value into the gain configuration field.