I'm pretty fond of the Gateway as a campus bar. The big windows let in a lot of sunlight, the open layout makes it easy to track down your buddies and the food is surprisingly good. Like, really, really good. It's true that you always, always get ID'd, but then I've noticed that Calgary is cracking down more in general.
There are TVs all over the place, so it's a good place to sit down and watch the game. They've got pool tables and some nice bathrooms in the back. The sound system is great for shows. It has all the elements of a great campus bar. There's always groups of friends crowded around, enjoying the specials. I feel like the Den gets rowdier but the Gateway gets points for usability.