To identify a GMC transmission, go to a dealer or mechanic's website that offers diagrams and specs of GMC transmissions. These sites show you what the part looks like and display the part number. Alternatively, go into a dealership that sells your make and model of vehicle, and ask the service department for help.
Many dealer websites include identifying charts that include the years in which the transmission was manufactured, the case type, and the make and model of the vehicle associated with the part. The charts may also list the vacuum modulator location, whether or not the transmission has a kick-down cable, and if it can handle a heavy weight and a high torque.
Photographs and images are labeled with measurements and part numbers, and some sites include transmission identification by pan, showing the shape of the pan, the number of the part and how many bolts are in the pan. Other sites offer a visual side-by-side comparison of similar parts.
If possible, first go to the dealership site for your specific vehicle, as it has the most accurate, up-to-date information. If you have an older vehicle that is no longer being manufactured, try a website or dealership that specializes in old or antique vehicles.