If you have an automatic with overdrive then there is a good possibility that it is the 700R4 transmission.
Read more: What_transmission_is_in_a_92_GMC_1500_with_a_350_or_what_transmission_might_be_in_it
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.