It has a 4L60E and that is the ONLY one that will work in that year.
If you have an automatic with overdrive then there is a good possibility that it is the 700R4 transmission.
If its a 305, toward the rear left of the air filter cover
If it is a 4 speed automatic with OD it may be a 700-r4
nearly any trans made by general motors will bolt to a small v-8
700r4
The 2011 GMC Sierra-1500 has a 4-speed automatic.
The 2003 GMC Sierra-1500 has a 5-speed manual.
The 2007 GMC Sierra-1500 has a 4-speed automatic.
The 2012 GMC Sierra-1500 has a 4-speed automatic.
The 2006 GMC Sierra-1500 has a 5-speed manual.
The 2004 GMC Sierra-1500 has a 5-speed manual.
The 2008 GMC Sierra-1500 has a 4-speed automatic.