Unlike the rest of the junkies here. The actual meaning is a "Large bag of gold" The reference "sunshine in a bag" is a quote from a Clint Eastwood(hence the name of the song) movie The Good, The Bad and The Ugly. Also one of the songs in the movie was reproduced and serves as the back-beat to the Gorillaz verison
Clint: Don't know
Kid: You gonna be alright
Clint: Yeah Kid, I got sunshine in a bag.
Joe: Yeah, but it didn't work out. She wanted me to give her sunshine in a bag. Too kinky.