I've heard that this a pretty common thing with the bumper guards. They're made to fit OEM bumpers. The aftermarket bumpers are a little bit different in the contour, I think to avoid having to pay huge royalties to GM for licensing fees. I still have my OE bumper, as well as my new chrome re-pop. I just ordered a set of guards this week, they're supposed to be here on Tuesday. I'm curious to see how the guards fit each bumper, and I'll snap a few pics for comparison, and post them.
I've heard that others have gingerly ground them to fit, sealed the ground off areas against rust, and used a rubber gasket, like a split piece of vac hose between the guard and bumper to disguise any minor gaps remaining. HTH.