First things first, I loved the KG trade, and wouldn't change a thing. The trade has delivered us three seasons of outstanding basketball, and a title. It's hard to ask for much more than that.
That said, for those of you who remember the summer of 2008, there seemed to be two big men who were available to the Celtics: KG and Pau Gasol. There was some speculation / rumor that we could have gotten Pau without giving up Al Jefferson; the trade would have been something like Theo + Gerald + Gomes + #1.
In hindsight only, would you do that trade instead of the KG deal? With KG's injury, it appears that Pau has a lot more left on his tires than KG does. In addition, we'd have another elite offensive big man on the roster. Inversely, our defense would be worse, and with our payroll structure, it's unlikely that we'd be able to be players in free agency (or that we'd have been able to sign some of the free agents we've been able to.)
The other thing to keep in mind is that if Pau is on the Celtics, that means he's not on the Lakers. Of course, that could have led to LA trading for KG, as well (although Minnesota didn't seem enamored by Bynum at the time.)
Even in hindsight, I don't think I'd reverse things. It's been too much fun with the "big three", and a bird in the hand is worth two in the bush, as they say. However, in an alternate universe somewhere, I wouldn't be shocked if the 2010 NBA champion Boston Celtics are hoisting a trophy with Pau Gasol as their playoff MVP.