Gruyère is pretty tiny. It's basically just the castle. It's pretty nice though. What time of the year are you going to be visiting? If you're visiting in the next couple of weeks you should be fine, and it will be light out into the evening. Though, by the time you get out of the airport and to Gruyère, it'll probably be close to 7PM.
Traffic can get bad between Bern and Zürich, so you'll want to heed
catandmouse's advice and get an early start to Zürich if you stay in Montreux. Or you could go have a quick look at Gruyère on your way to Zürich and spend the night in either Zürich or Luzern.
You don't have a lot of time, but you'll still manage to see some nice countryside. It's a nice place here in Switzerland.
