Make a region in front of the gate. I'd call it gate GateOpen.
Then make a new trigger:
Events
Unit - A unit enters GateOpen
Conditions
((Owner of (Entering unit)) is an ally of Player 1 (Red)) Equal to True
Actions
Destructible - Kill (the gate you want opened)
Wait 5.00 seconds
Destructible - Resurrect (the gate you want opened) with (Max life of (the gate you want opened)) life and Show birth animation
This trigger assumes that P1 and his allies are the ones who you want to be able to open the gate. Change 5 secs to whatever you want but i find 5 secs is best