how do i set a trigger for lowering gates... and keeping them up when i want them too. so like they have to kill something before the gate goes down... can someone tell me what i have to do to make it work...

Disabling the doodad state of a door will make it go down, enabling it will make it go up, it's that simple.

All players bring at least 1 men to gate / disable doodad state for anyunit for P12 at gate.
All players bring at most 0 men to gate / enable doodad state for anyunit for P12 at gate.

And the condition for having killed the unit you want before it opens.

ok nothing, it is just too late.

Triggers:

Trigger("All players"){
Conditions:
Score("Current Player", Kills, At least, 1);
Bring("Current Player", "Men", "Gate", At least, 1);

Actions:
Set Score("Current Player", Set To, 0, Kills);
Set Doodad State("All players", "Right Upper Level Door", "Gate", disabled);
Preserve Trigger();
Comment("Open");
}

//-----------------------------------------------------------------//

Trigger("All players"){
Conditions:
Bring("Current Player", "Men", "Gate", Exactly, 0);

Actions:
Set Doodad State("All players", "Right Upper Level Door", "Gate", enabled);
Preserve Trigger();
Comment("Close");
}

//-----------------------------------------------------------------//

Where it says "Right Upper Level Door", you can just switch that out for the exact name of the doodad you want to have.