Marc Gasol's value at the time was much, much lower than Al Jefferson's value.
Also, I don't think you can so quickly dismiss the extra players in each deal as 'equal'. Javaris Crittenton, Kwame Brown, Aaron McKie vs. Ryan Gomes, Sebastian Telfair, Gerald Green, Theo Ratliff. I'll call McKie and Green a wash - a player who is now out of the league vs. a player I hadn't even heard of before looking the deal up now. Kwame vs. Telfair is a wash - two busts who now serve as mediocre bench players. Crittenton, after looking at his tools, has potential but isn't getting any minutes for a Washington team stacked with guards. Meanwhile, Gomes is a borderline starter and Theo Ratliff is still a decent shotblocking big off the bench. I'd say the pieces we gave up are more valuable than the pieces they gave up, and I don't really think it's that close.
In the end, I still think Gasol was much more of a steal than Garnett.