Understanding the statement: An antenna with 20 degree beamwidth has 20 dB gain
Study the following document: https://www.phys.hawaii.edu/~anita/new/papers/militaryHandbook/antennas.pdf
which discusses basic antenna theory. Equation 6 is what you want:

Antenna Gain (dB) = 10log(41253/VH) where V is the vertical beanwidth and H is the horizontal beamwidth (both in degrees). If you insert V and H as 20 degrees, then, indeed, the Antenna Gain is 20 dB.