My '85 Sierra Classic doors are both like that. About 3/8" from closing tight at the bottom near the cab.
Its always been that way and has all original sheet metal never disturbed. But, it can be adjusted out. The top of my doors are in about 1/4" to far. Just a matter of rotating the door on its hinges.
Although I've never done it on a truck before, I've done it many times on several Chevelles and Camaros, and the hinges are the same design as our trucks. The adjustments are huge on these things!
I've got a really good article around here somewhere on panel alignment. I'll see if I can find it and post it up. Lots of good info in it.
I'll get a few pics of the gaps on my truck too.