This would go in map developtment, but since you posted here ill answer here. Just make a trigger "pick all units owned by nuetral hostile and do order picked units to attack gate". Sorry thats not exact, but since im not sure how many things we are talking about and everything, it will have to do. And if you only want a certain few to attack, make a region with them in it and do a "pick all units in region and do if owner of picked unit is equal to neutral hostile then do Order picked unit to attack gate." Thats for a region and a player. | 
