They're not getting rid of Kepler because he is due $10 million. They are spending slightly more on Gallo for an upgrade of nearly 50 points of career OPS, and an increase in power (Kepler 1 HR every 22 ABs, Gallo 1 HR every 13 ABs). Both are nearly equal defensively, with Gallo having 2 Gold Gloves, and Kepler being a finalist a couple of times. They don't need 2 guys like that on the roster, hence they trade Kepler.