If it's an automatic, it'll have the TH-350 (C/K 10) or the TH-400 (C/K 20 and 30). If it's a manual, you probably have the SM-465.
The 2008 GMC Sierra-2500HD has a 6-speed automatic.
The 2006 GMC Sierra-2500HD has a 5-speed manual.
The 2014 GMC Sierra-2500HD has a 6-speed automatic.
The 2012 GMC Sierra-2500HD has a 6-speed automatic.
The 2008 GMC Sierra-3500HD has a 6-speed automatic.
The 2007 GMC Sierra-3500HD has a 6-speed automatic.
The 2013 GMC Sierra-2500HD has a 6-speed automatic.
The 2011 GMC Sierra-1500 has a 4-speed automatic.
The 2001 GMC Sierra-2500HD has a 5-speed manual.
The 2011 GMC Sierra-2500HD has a 6-speed automatic.
The 2002 GMC Sierra-2500HD has a 5-speed manual.
The 2003 GMC Sierra-1500 has a 5-speed manual.