LOL Greymane close the gates because of they wanted nothing to do with the alliance during the second war. They believe the alliance needed them more than they needed the Alliance. They think they can exist by themselves. They were absent through out the third war because they gates were close so they were unaffected by the Scourge. The gates were close prior to wc2. You rread the lore and play wc2 and wc3 back.
If he didn't made the selfish decision, Lordaeron would still stand, but yet the Forsaken emerged as the true hero where they manage not just to break free from LK's mind control, but also the land from the Burning Legion.