This doesn't increase gain, it just takes it from the back and adds to the front.

"Gain" is routinely used in the world of RF to refer to the effect of directional antennas (either as dBi (relative to an ideal isotropic radiator), or, more rarely, as dBd (relative to a plain dipole).

One of the ways the FCC specs power allowed is by field strength at a given distance. I can't remember if this is how ISM band (all 802.11) allowed power is specced.