We actually have several threads open about this very topic already. In short, the flat earth society don't believe that gravity does vary. We think its a constant. (actually we don't believe in gravity at all, but instead universal acceleration due to the equivalence principle as first explained by Albert Einstein, but that's another story).
Please read around some of the other current threads. Examples such as gold prices, aircraft fuel loads and how much you weigh on holiday are currently being discussed in relation to this.
Oh, and welcome to the flat earth society.