Good news: He catches what he gets to, most of the time. His arm isn't horrible-- accurate, though a bit short. He has a nice, quick release on his throw. He isn't afraid of the wall and plays the ball well on a carom.
Bad news: He has no instinct for OF play. He reacts to the ball very, very poorly and has absolutely no catch-up speed. His routes are probably the worst I have seen from anyone in AAA in 20 of watching minor league ball. He circles balls rather than going at a line (which adds to the appearance of his being slow). Too, he sometimes airmails throws to the cut-off (though he's better at hitting the cut-off than is Sappelt).
In short, Gomes is the better OF and by a fair margin.
Now, Alonso might improve. His route-taking has already improved some this year over last. His OF speed is also improved. But the best he can hope for is a Ryan Braun/ Greg Luzinski/ Adam Dunn type of glove out there.
Basically, he's a DH with a glove on his hand.