I would definitely trade Pierce for Gasol. Gasol, KG, JO, Bass and Stiemsma is a pretty [dang] good front-court and the Celts desperately need another big man.
Right now, small forward isn't as much of a need as Pietrus and Daniels can play the position.
Not to mention, there will be a TON of very good small forward free agents next summer (Gerald Wallace.)
I think this trade benefits the Celts in the short and long term.