Gantry is a very sophisticated theme framework. It basically allows you to build sophisticated themes without all the ground work that a regular theme would require. It’s kind of independent of the CSS framework, you can use whichever one of those you want.
However, don’t get too tied up in the CSS framework, it’s just that a framework to build your own stuff on top. Most theme designers prefer a simple framework that provides the bare minimum so it doesn’t have the overhead of a big framework with features that you need and therefore don’t have to go to the trouble of overriding.