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@mp775: the link you give about the identification of the bus (http://www.angelfire.com/ny5/nebuses/rosters/lacmta1.html) says GM as make 
But if you look at Pages 2 and 3 of his Los Angeles roster, he also uses "GM" not only for post-1968 New Looks, but also for RTSes:
http://www.angelfire.com/ny5/nebuses/rosters/lacmta2.html
http://www.angelfire.com/ny5/nebuses/rosters/lacmta3.html
Meanwhile, his Oceanside/North San Diego roster lists everything as "GMC," including Old Looks back to 1956:
http://www.angelfire.com/ny5/nebuses/rosters/northcounty.html
I read what follows as a pro "GM only" argument. At the bottom of page of Yellow Coach Part 2 (penultimate paragraph), in CoachBuilt.com (underlining is mine):
That description says to me that Yellow Coach was merged into GMC.
When searching literature about GM Coaches (manuals or -less present- brochures), one can find GM Coaches logo on documents about for pre-1967 buses, and GMC Coaches for 1968+ models... i.e. for the ones which have the GMC logo appearing in the front face of the buses.
And on this service manual, it says "General Motors Corporation" on the emblem between the GM and the COACHES.
I see it as "GM Coaches, part of the GMC Trucks and Coaches Division". And it became "GMC Coaches, part of..." in 1968. The same way GMC trucks are badged... GMC trucks, not GMC Trucks and Coaches Division, which appears only during the 1970s:
GMC trucks were built by the same GMC Truck and Coach Division during the pre-1968 era:
1955 GMC Truck parts book
1957 GMC Blue Chip pickup ad
In my mind, we should give these buses the GM make from 1943 to 1968, and the GMC one for 1968+ models.
Since there was no change in the corporate structure in 1968 and no material change in the vehicles themselves except for a slight change in the emblem, I think it would only be more confusing to separate them in the database. It also would make "GM" less useful as the catchall for trucks that can't be identified as either GMC or Chevrolet.