directional dependence of antenna gain

the gain of the antenna has directional dependence ten how do we simply define it by one number.like for half wave dipole the antenna gain is ~2.15db.

do we measure the antenna gain all direction and then simply take the directive one as the antenna gain.

Good question.
If it is a directional antenna, common sense would dictate that at it's highest gain would indeed be the number used.
Just like you would only use the Gain directly in front of a parabolic, as that is the one (and only) you care about.

"do we measure the antenna gain all direction and then simply take the directive one as the antenna gain."

I'm pretty sure on directional antennas that is the convention.