What exactly is (Horn) Antenna Gain

I was doing an experiment for finding the radiation pattern of Pyramidal Horn Antennas at RF frequencies(as part of my course curriculum). There I came across the Friis Transmission Equation

What I don't understand here is the two terms Gt and Gr. What does these gain terms refer to? Gain is something associated with active devices like amplifiers. But how come antennas have gain? They don't have any source of energy so that they can amplify the power, they are transmitting or receiving.

-

The gain is an indication of directionality. IF you have an isotropic emitter that sends , say 1 W, into complete sphere you are emitting radiation over $4\pi$ steradians solid angle. If you make the antennae more directional and emit over a smaller area then the energy is concentrated (per steradian) in a smaller solid angle. This increase is a gain, gain in an active amplifier is a dimensionless number, and so is this.
Emitting radiation over $4\pi$ what? –  Phil Frost Aug 10 at 11:46