Also, there's a problem with selecting against high gain% cards. You can't just keep them out of the pool, so there will probably be at least 1 on the table. Since the other cards will generally be of lower power level, the high gain% card is even more likely to take over the game.
If you have Gain% data and want to build a kingdom chooser around it, the answer is to select cards of the SAME gain% strata. That's how you avoid the one-dominant-card syndrome. Get a kingdom of cards all with similar gain% and you'll create real choices. At the upper end, you might see KC, Bridge, Mountebank, Fishing Village, Chapel all together, and then you do have real choices even here on _which_ of the strong cards to get first and build around. Or a set centered around lower gain% could have stuff like Apprentice, Smugglers, Workshop, Jester, Cutpurse together... cards that get overshadowed by the bullies, but could shine when playing in their own weight class.
Of course there's a feedback loop of gain% data, such that the big cards will decrease in gain% over time as they compete against other strong cards, and middling cards will pick up some more attention. So we narrow the spread in gain% such that all cards get picked similarly often? To that I say Mission Accomplished.
I'm very much not sold on the idea of picking always at least one attack, one village, one trasher, one alt-VP, etc. Dominion has a richer space of possible games when sometimes you can build unmolested to your heart's content, or terminal slots are precious and must be carefully chosen, or you must deal with the starting coppers/estates, and so on.