The problem is that the type of gradient you are using screams "photoshop/gimp" which means it screams "after year 2000."
The original coverart is obviously watercolor-heavy. If you really want to pay homage to it...
...try printing the basic layout VERY light on a piece of thick paper, maybe card stock. Then paint in the gradients with watercolor, something basic. Then try to blend and merge your watercolor Hint:avoid black, use something like indigo violet, then achieve black later with digital adjustments.
If you're not comfortable imitating the watercolor and airbrush style with actual watercolors, drop the attempt to imitate it. Basic gradients scream amateur, but your title screams scream, "pro." So, maybe drop the gradients altogether and re-approach that box art in a different way.
(In fact, ideally, I would print the boxart really light and watercolor all of the art, including the ballplayers. However, if you don't have experience with watercolors or painting, I could see that being a touch too intimidating.