This vehicle is similar to the Chevrolet Express van, which it shares a platform with.
With a wheelbase of 135.1 inches, it is one of the largest vehicles made by GMC. It is distinct from most other full size vans because it has abnormally high taillights, which are a safety feature. When the rear doors are open, which they might be if the van is carrying long cargo, the lights can still be seen from behind.
These larger vans tend to get heavy use by commercial businesses. There are still plenty of these out there in decent condition. If you can't find any in your local car classifieds, try looking on eBay. There are many fleet services that sell their older models on there, often at steep discounts (it beats going through the hassle of listing them in classifieds and waiting months for buyers to show interest).