The disabled condition should be removed altogether, it is not relevant and will only cause the trigger to fail if you enable it.
The error that I see is that you use "(Owner of (Ordered unit))". "(Ordered unit)" is an event response that can only be used in triggers with an order event. You should use a different method of picking the new owner. Also, make sure your power aura can also target neutral units or you will never be able to power a Stargate again once it becomes unpowered.
Edit: I've edited your post and put the trigger in [trigger] tags, that way it is easier to read.