why not just filter for gravity for each category to find out what is promoted the most
Problem with this is, it doesn't always tell you what converts the best. It just tells you what is getting promoted a lot.
Something may convert terribly, but if enough people promote it, it will get a high gravity. This is ESPECIALLY true of products going through a launch.
If you're going to search products by gravity to find a good one to promote, I'd suggest finding an older one that is still up there in gravity. Reason being, if it's past all the hype of a launch, there aren't likely as many people promoting it. If it's still competing in terms of gravity, despite fewer people promoting it, then it probably converts well.
More important, I think, is evaluating the sales copy, product itself (ask the vendor for a sample), landing page, price point, etc. If you know your audience, then you'll know what will convert, regardless of gravity.