Nobody's paying attention to you because +/- is a dumb statistic. It incorporates everybody else that Pau's on the floor with and not just his very own play when that's exactly what we are evaluating... Pau Gasol's very own play.
You still sound confused.
First, the Lakers have a better +/- when Pau is on the court than off.
Second, lets look at the performance of Lakers 5 man units.
The most prevalent 5 man unit (192.6 min) has been Morris-Bryant-Metta-Gasol-Howard. That unit has a Win% of 61.5% - meaning they have consistently outscored their opponents.
The next most prevalent 5 man unit (118.3 min) has been Duhon-Bryant-Metta-Jamison-Howard. That unit has a Win% of 40.0% - meaning they have consistently been outscored by opponents.
Most of the 5 man units that Gasol is in have >50% Win%.
Gasol is clearly not performing at his levels in the past but its not the same team either. When Gasol isn't on the court the Lakers are getting schooled by teams. Why trade him?