I bought my GMC in 2007. It's equipped with the Duramax and Alison 6 speed Auto Transmission. Wife doesn't drive a manual transmission vehicle and I'm not going to try to teach her. A year later, found out only way to get the standard trans with the Duramax was to special order it. Have no idea if it's still available with special order or not.
Had not shopped or owned a pickup for several years (spent a lot of time overseas). Supposedly automatic transmission is better for towing/etc. Not sure I agree it's better but have learned to like it. Especially towing 5th wheel thru mountains.