It's not entirely clear to me where this casino would come - inside the town which as people are saying is already touristy and not particularly authentic or interesting? In that case, it doesn't seem like there'd be much damage to the battlefield or to its reputation. That is, unless the casino starts using the battlefield in its advertizing, which one might argue they'll do automatically through the name alone, even if they don't otherwise focus on it (which one assumes they probably would). So eh, if it were up to me, rather not.

Whereas if it's really located close to the battlefield, I'm opposed a fortiori.

I live within 20 miles of a rather famous battlefield myself - Waterloo. Now, that one has certain elements that one might call tacky as it is (there's an artificial hill with a statue of a lion on the top, symbolizing one of the Allied princes who supposedly got wounded on that location but kept fighting anyway, something like that), but if someone had the idea of building a casino or something similar there... yeah, I'd be opposed to that too. And I think Gettysburg is far more important to the national sentiments of Americans than Waterloo is to that of Belgians (after all, Belgium was only created fifteen years after Waterloo).