Here's some tid bits for ya, mostly good news. (I won't reveal my source or any specific info that would damage GM's competetive stance):
2014 trucks (release first quarter 2013)
New non-turbo V6 across the board (likely LFX), around 315 Hp
6 speed auto across the board, inc reg cabs, V6, etc
This will give class leading MPG..... (25+mpg??? Hint Hint)
4.8 liter a goner
Gen V V8's: 5.3 liter, 350hp, more efficiency (DI, gears, etc)
optional 6.2 liter, 425hp
*E-assist coming to trucks*
The best part: the Sierra looks PHENOMINAL and will blow people away, and doesnt look like a chevy, which will be the more "work truck" kinda feel and more "vanilla". Sorry Chevy guys. GM is aiming to increase GMC sales ALOT. Yukon/Tahoe is said to look several generations ahead of the current ones. Will also blow away competition.
Only thing I'm dissapointed with about the news is the low HP figure for the 5.3 liter. It needs to match the Ford 5.0.
*Please keep in mind this post is from 09-29-2011! Some of the insider source information has changed/been updated as GM has continued the development of the finalized truck. There is over 100+ pages of discussion for you to enjoy