I wouldn't call a 6 year career aveage of 42% from the feild being one of the best shooters in the league. I wouldn't even call that mediocre. I'd call that pretty bad actually.
18PPG for 14 seasons isn't enough to get into the hall of fame... If he can stay consistent, which has been a problem for him, AND add to his game, which I think he very well can, he has a chance. I'd like to see him make better decisions with the ball. If he can shoot more efficiently, and take advantage of his lenth and become more consistent in rebounding and especially defense, then he could very bell become one.
BUT, if you extrapolate his current production for another decade, I think he fall well short of HOF.
He's more a future Peja Stojakavic in my opinion. Not that that is a bad thing, I love Peja, but HOF inductees should be able to consistently play at that next level. Not sure I would say that about Gallo.