On long rides, I fill my water bottles with Gatorade, and I put water in a Camelbak. I also carry and eat energy bars.

On your question about Gu - it's a matter of personal preference. I find that Gatorade (or the equivalent) plus snacking on energy bars - or fruit - works for me. The key for me is to make sure I'm eating a little bit of something all of the time. That is, I don't ride for two or three hours and then eat something big - instead I eat small bites all day.