AFAIK, if the player has planets very close to the DS, the Gatlings focus almost exclusively on filling those planets with patrolling ships. They only seem to go on the attack if there are no nearby friendly planets, or if there's a massive Threat buildup near one of friendly planets they're patrolling.
It might be that the Gatlings do that if there's only one friendly planet to run through, as they think there's nowhere else to go. Presumably, if an AI wave decides to attack that planet, they'll 'wake up' and help defend, only to return to endlessly patrolling the wormhole, once the coast is clear.
If you take another planet nearby, they might start behaving more normally.