"The Gilded Six-Bits" is a reference to the supposedly gold watch chain that Slemmons wears. What exactly is a gilded six-bit, you ask? Good question. To start, gilded means something overlaid or covered (in this case, the half-dollar) with a thin layer of gold or a gold color. A bit is an amount we don't use anymore, but originally was worth about 12 and a half cents.
In the context of the story, the six-bit represents appearance versus reality, and the danger of getting caught up in material possessions. Slemmons, the guy everyone thinks is so great, turns out to be a fraud and almost ruins Joe and Missie's marriage. Before he came into the scene, they were perfectly content with the little money they had.
The title also brings to mind desire. Hurston puts her critical writer's eye on the desire for money, for things like clothes and jewels, and even the desire by blacks to be white. Judging by the events of the story, desiring that which you don't have is not a very positive thing. The grass is certainly not always greener on the other side.