Agreed. Always wondered why they went from 400 to 800 though.
GMT codes are used for EVERYTHING that GM considers a truck or truck-like...SUV's, s10's, minivans, crossovers, vans, etc...and there are many different segments of the GMT platforms...the full size trucks just happen to be 400, 800, 900 (ish)....for example an express van is a GMT600...there's even segments within the GMT platforms...like a 99-06 Suburban is a GMT830, where a tahoe is a GMT820...newer ones are even more segmented, a current silverado (crew/extended/regular cab) is a GMT901, where a silverado HD is a GMT911, and a Sierra is a GMT902...
getting super specific like that isn't really needed, but using the main 400-800-900 terms is very helpful vs the OBS/NBS stuff...
here's a link with some of the GMT platform designations if you want to read up on it:
http://en.wikipedia.org/wiki/GM_GMT_platform