Gains Mod, or a multiplier on the amount of base powerlevel you gain per point of damage, can be affected by many things:

Race Gains: The inherent growth of your race.

EQ Gains: Increased by equipment, particularly rare or weighted items.

Double Gains: Earned through Quest Points; permanent effect.

RPP Gains: By role-playing and spending RPP, you may purchase boosts.

Global Gains: Increased by the amount of players online at once.

Double Exp: An Immortal gift that doubles pl gains. Do not request.

Enhanced Gains: A donation bonus that increases combat gains.