Originally Posted by stoffe
Hmm, are you sure they re-open? In my current game I've closed 56 Gates and I've run out of gates to close. I've looked all over the place and the only active Oblivion Gate I can find is the one that leads to the Shivering Isles.
Perhaps they will re-open and more appear if I progress further in the main quest?
According to the game guide, the further you get through the main quest, the more chance you have of re-opening gates. I've never looked into it myself. They all close permanently at the end of the main quest though.