Unless you're talking about packing on a passel of pounds before your weekly weigh-in at the Overeaters Anonymous meeting, the word GAIN generally has a positive connotation. When talking about antennas, the word refers to its directionality, as compared to another antenna, like a dipole. (a very basic antenna consisting of center-fed wire or tubing)
In simplest terms, when it comes to antennas, size does matter, and bigger is generally better. The larger the antenna, the higher the gain, and the higher the gain, the better you can send and receive signals in a specific direction.
Adding additional directors to a Yagi increases its gain. That's why the antennas used for such things as moonbounce and direction finding have so many elements. Same for cell tower, and TV antennas.
So to amateur radio operators, GAIN is a very good thing. (Don't even bring up that nasty word loss.)
Oooh, such fuzzy reception! I think that guy needs a higher gain antenna.