Why Ground gain is the extra signal strength that comes from the ground acting as a reflector. Energy radiated downward bounces off the earth and, at certain elevation angles, arrives in phase with the direct wave, adding to it. Over a perfect reflecting surface this addition can theoretically reach 6 dB, and over real earth or salt water HF antennas commonly see roughly 3 to 6 dB at low takeoff angles. Nothing about the antenna itself changed; the environment supplied the gain.
Watch out The choice about grounding the antenna confuses ground gain with a DC or safety ground connection. Ground gain has nothing to do with wiring the antenna to earth; it is purely a reflection effect, which is why height above ground and ground conductivity matter so much on HF.
Ground gain = free bonus from the mirror below you, up to 6 dB when the reflection adds in phase.