What is meant by Antenna Gain?

Antenna gain is a measure of directivity of the antennas radiated signal. Remember antennas are a passive device and will never put out more total power than is put in to it. An antenna with gain focuses the power in a particular direction at the cost of reduced power in other directions. This is what gives an antenna gain. The simple fact is the mounting location has more affect on the ultimate gain and radiated pattern than the antenna type does.