For Gravity Warhead, it's in BuffGravityWarheadSpawn.entity:
numInstantActions 1
instantAction
buffInstantActionType "ApplyBuffToTargetsInRadius"
instantActionTriggerType "OnDelay"
delayTime 0.000000
buffType "BuffGravityWarheadAction"
targetFilter
numOwnerships 1
ownership "Enemy"
numObjects 2
object "CapitalShip"
object "Frigate"
numConstraints 0
range
Level:0 3000.000000 < Level 1 range
Level:1 3500.000000 < Level 2 range
Level:2 4000.000000 < Level 3 range
For Volatile Nanites I'm not sure which of the 2 you'll want. The first is the radius for ships affected by the nanites in BuffVolatileNanitesSpawner.entity:
instantAction
buffInstantActionType "ApplyBuffToTargetsInRadius"
instantActionTriggerType "OnDelay"
delayTime 0.000000
buffType "BuffVolatileNanitesTarget"
targetFilter
numOwnerships 1
ownership "Enemy"
numObjects 3
object "CapitalShip"
object "Frigate"
object "PlanetModule"
numConstraints 0
range
Level:0 2000.000000
Level:1 2000.000000
Level:2 2000.000000
The second is the radius for the damage dealt on ship death in BuffVolatileNanitesTarget.entity:
instantAction
buffInstantActionType "ApplyBuffToTargetsInRadius"
instantActionTriggerType "OnOwnerDeath"
buffType "BuffVolatileNanitesDeathDamage"
targetFilter
numOwnerships 1
ownership "Enemy"
numObjects 3
object "CapitalShip"
object "Frigate"
object "PlanetModule"
numConstraints 0
range
Level:0 1000.000000
Level:1 1000.000000
Level:2 1000.000000