I'll start with the mana, because it's pretty obviously the core of the deck.
Four colors is rough no matter what you do, but I'd throw a set of Darksteel Ingots in just for generic mana-fixing. Considering all your important gates enter tapped, I'd say you should replace at least some of those gates with Vivid lands, too. Say Vivid Grove. They enter tapped, too, but they can give you any color mana twice. Obviously, it trades late-game functionality for mana when you need it, but you will have the mana you need when you need it.
Other things? I really don't see much card-synergy in this deck aside from a few gate sorceries, and certainly not that warrants the trouble of FOUR colors. Right now you have all four colors in both the deck and the side-board. I know what I'd do: sideboard one color at any given time.
As green and white appear to be this deck's primary colors, roll with that. Set up two sideboards: one black and multicolored, one red and multicolored, and any one game you play one of those colors sits out. You'll still be running three colors, so there's plenty of space for gates, you can fit some basic lands in to give your deck an early mana boost, and the whole deck will just deploy more smoothly if it just has to worry about three colors.