Gallo was 21!!!!!!!! Baez is a great example. Came up at 21 and struck out at a 41% clip IIRC. He's now 23 and has a 23.4% k rate. Baez has insane power for a middle infielder, but most would agree Gallo has more raw power.
In 36 games as a 21 year old he had a .6 WAR. If you extrapulate that over 500 PA which seems to be a reasonable average that an MLB regular plays, that ends up with about a 2.5WAR. And this is at 21 with a sky high k%.
Even a slight adjustment to the mean, think a Sano level (33%) and he is pretty easily over 3 WAR and closer to 4. If you want a 1st baseman that gets on more, don't get Gallo. It's pretty cut and dry. Plus he had more value as a 3B since his arm is so good.

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