Gravity is not always the telltale sign of whether the product "sells" or not. I think its best to actually look at the quality of the selling page and see if you can get a copy of the product to check the quality. You need to try and think about things such as refund rates, not just the initial sales you could make.
Going to the unknown sometimes could be profitable but i think it is more profitable when you stick with the known
Why not look at what Clickbank says themselves? According to a posting I read on their blog, the most consistent money makers are those products with a gravity between 40-100. Those products with a Higher gravity may sell well at first but eventually burn out; if you want to SEO for traffic, why not focus on those products that sell over and over and over? A perfect example is this "phone search" product. Consistently high gravity over three or four years....