I've never seen a GMC manual, so I don't know. I have a '68 C/10 stepside and '67 K/10 Subn --both Chevy -- and a '71 GMC Jimmy. The '71 chevy manual I bought covered the '71 pretty good. [The '67/'72 Factory Assembly manual does have a lot of GMC drawings in it too, but this manual is really a collection of looseleaf RPO mods sheets sent to dealers. It has no index, or part numbers.] For this vintage-series, the chassis and overhaul books are mostly interchangeable. The '67 and '68 manuals are mostly identical too, except for the sm420 vs sm465 xmsns and no side marker reflectors on the '67. GMC wiring is slightly different since ther're 4 headlights instead of 2.
I would be interested in seeing a GMC CS&O/H manuals set, but for practical purposes a Chevy set of manuals is 95% effective.
Semper Fi, Do or Die.