Gordon's the worst due to total lack of production (not even playing)
It's a toss up between the other 3 tbh. You'd have to measure production to cash ratio, so even though JJ is possbily the best player of the other 3, he costs a ton (33% more than Boozer, 100% more the Gallo). Boozer is better than Gallinari, but costs 50% more. Just comes down to preference then.
But if I had to pick, I'd put JJ as the worst due to 80m owed, then Gallo, then Boozer. At least Boozer's deal is almost up and he's still good on offense (and not as bad as blamed for on D'). Next season he should be tradable, and the offseason and season after that he's easily moveable for a solid piece. The other guys you're just having to choke on the salary for a longer period of time and getting crap production.