Gain

Read the full line after Gain. "Gain 20% of Damage as Extra Fire Damage" adds Fire Damage equal to 20% of the applicable base damage and keeps the original damage. A line that instead gains Life, Mana, Armour, Energy Shield, a Charge, or an extra Skill cost is not damage gain and follows the named resource or stat.

Damage gain keeps the original damage

Gain conversion adds damage of the output type without removing damage from the source types. Regular conversion replaces part of the source damage, so the two wordings do not produce the same result.

The output type controls scaling

Damage gained as a specific type scales with modifiers that apply to the gained output type, not modifiers that apply only to the source type. Physical Damage gained as Lightning Damage therefore scales as Lightning Damage, not as Physical Damage.

Gain uses base damage before increased and more modifiers

Damage gain is calculated from the applicable base damage before increased and more damage modifiers. The gained portion and the retained original portion are then modified according to their own final damage types and the Skill's other applicable modifiers.

Skill gain happens before other gain and conversion

Damage gain inherent to a Skill is part of the first conversion step. Gain and conversion from global effects, items, buffs, Supports, and passive skills use the second step after the Skill's inherent result.

Damage over time cannot gain damage

Damage over time cannot benefit from damage gain. A Hit can gain extra damage and may then create another effect under that effect's own rules, but the damage over time component does not independently gain an extra damage type.

Scope words restrict the gain

Words such as Attacks, Spell Hits, Allies, enemies, while, per, and against identify what receives the gain and when it applies. A global damage-gain line does not have the same scope as a line limited to Attacks, one Skill, one target state, or allies in Presence.

Other Gain wording is separate

Gain can also add a Charge, resource, defence, threshold, or cost. These lines use the named source and result directly and do not inherit the damage-conversion rules merely because they contain the word Gain.