This is really a no brainer...
Gasol is a 7 footer who can impact the game just as much on the defensive end as he can on the offensive end.
He's one of those things that is rare these days...an old school center with an all-round game. He can defend the paint, he can score in the paint, he has a decent jump shot, he's an excellent passer, a good rebounder, and he's a proven leader.
Kevin Love on the other hand is major defensive liability who can only excel on a team that is filled with enough defensive talent to cover up his flaws. This is a big reason why Cleveland are struggling right now - their defence (with Kyrie and Love) is atrocious...and scoring 25 points every game means nothing if you're also giving up 25 points.
Mark Gasol has led a number of teams deep in the playoffs. Love hasn't even led a team TO the playoffs.
It's not even debatable, you'd be crazy to choose Love.
Plus to make this even more one sided, PF is currently our most stacked position (Sully, Olynyk, Bass). It's arguably our strongest position (along with PG). Center on the other hand is undeniably our weakest position, and the one most in need of improvement.
There is no doubt at all that we would be a far better team with a Sully/Gasol or Olynyk/Gasol frontcourt then we would with a Love/Olynyk or Love/Zeller frontcourt.
Boston is actually 3rd in the NBA in scoring (PPG) at the moment. We aren't having trouble putting the ball in the basket. What we're having trouble with is stopping the other team from putting the ball in the basket. We need a player who can impact the game defensively. In fact when you consider that we already have Smart, Bradley, Rondo, Green and Wallace...we are basically one defensive center away from being an elite defensive team.