Gain is a Fallout 2, Fallout Tactics, J.E. Sawyer's Fallout RPG, Van Buren and Lionheart perk. In Fallout 3 and Fallout: New Vegas it was replaced by Intense Training. This is a series of perks (Gain Strength, Gain Perception, etc.) which will modify the selected primary statistic by +1. Each statistic can only be boosted once, and only to your racial maximum.