Mostly, antenna gain is misunderstood.
For example, if you have a mobile antenna you'd better off with a 0dBi antenna rather than a 10dBi antenna. The more gain the antenna has, the more energy will be bundled in one direction. Think of putting a reflector behind a light bulb. All the light hitting the reflector will be reflected, no light will go through.
For a mobile antenna you don't want 'black spots'. Effectively, you want your sensitivity/gain be the same in each direction (0dBi). This is called isotropic. Any misalignment in the pattern will yield gain in some direction. A dipole is not fully isotropic (radiating less in line of the dipole and radiating more perpendicular to the dipole), think of a radiation pattern like a squeezed ball.
Stationary (gateway) antennas will have more likely gain in horizontal direction and less gain vertical direction. This can be achieved by stacking dipoles for example.
An antenna can not output more energy than goes in. On the other hand, an antenna can 'loose' energy. When there's a mismatch in the antenna (system), part of the energy get's reflected and will not reach it's destination. This is measured in VSWR (or in S parameters). A VSWR of 1 is most ideal: no energy is reflected.