That depends on a lot of factors. Gravity is a measure of how MANY affiliates have made sales in the last X number of days or something. However that just says a the products have attracted a lot of affiliates and says nothing of conversion rates and other factors. You are best of doing research into a niche and checking out the sales page and making a judgment on how good that is. In the end it is all a bit of a gamble, you just gotta test what works best for your methods.
Like hengster said, high gravity means a lot of affiliates are promoting the product. This means greater competition, meaning more difficult to get good search rankings for newbies.