Not to derail the thread, but the Silverado and the Sierra are virtually identical trucks. They've only been stand alone trucks for the last twenty years, and while that might sound like a lot, I drive a 1998 K-1500 Silverado (with well over 300,000 miles, and it still runs great). The Sierra of the same year was only a trim option. And while they are separate trucks entirely now, they're more similar than not. The only real differences are cosmetic. They are even virtually the same price. A base model GMC Sierra is less than $1000 more than a base model Chevy Silverado, options and trim packages notwithstanding.
The only real difference between the two are production numbers. GMC produces substantially fewer trucks than Chevrolet. So, "flagship model" is a term that really doesn't apply. General Motors produces them both, and as far as I know, has never considered the Sierra to be the flagship truck offering. Chevy has their flagship, as does GMC, and the difference between the two is insubstantial. The parent company reaps the profits either way.