I've read that GMC used Oldsmobile and other non-Chevy GM inline sixes before the GMC motor came out. I don't recall if they also used Buick or Pontiac, but I know at least one other manufacture was used in some years. I also don't know exactly when GMC started producing their own motors, but I believe it was during the 30's.
You can probably get a real good answer if you visit your neighbor at oldgmctrucks.com.
