The serial numbers are classified by the following:

Length: New Looks
40 foot models: 5301-5310
35 foot models: 4516-4523

Length: Old Looks
40 foot models: 4501-4515
35 foot models: 3501-3510

The first production GMC New Looks were introduced in 1959. The first group (5301-5302) looked like this: front of a 5301:
http://www.nycsubway.org/perl/show?15981

rear of a 5301: Link to "images.google.com"

bus #438

The second group (5303-5304) was introduced in 1963, and was GMC's top selling New Look. They featured some minor changes (marker lights, tailgates, engines)

TDH 5303: http://www.motorbussociety.org/conventn/02spr/DTS%20494.jpg
rear: http://gallery.bustalk.net/displayimage.php?album=74&pos=25

The 5305-5306 was introduced in 1967, and featured a more standardized tailgate and look. These were very popular in San Francisco
5305: http://gallery.bustalk.net/displayimage.php?album=74&pos=37
5305 rear: http://gallery.bustalk.net/displayimage.php?album=74&pos=53

The 5307-5308 was introduced in 1972, and was very popular with Chicago, with over 2000 built over a period of four years
5307: http://www.busdrawings.com/Transit/illinois/chicago/1972t8h5307a/1233.jpg

The last group was the 5309-5310, which were introuced in 1973.
5309: Link to "www.busdrawings.com"

GMC closed their American plant in 1986, with the New Looks being produced in Canada under the Brown Boveri Industries.
1981 BBI ETB: http://www.busdrawings.com/Transit/alberta/edmonton/bbc/index.htm

The last GMC bus from Canada was a group of 26 1986 5307 'A' for Santa Monica Municipal Bus Lines
5307 with a rehabbed tailgate: Link to "www.busdrawings.com"
Thanks to Snookie

Actually, the Canadian GMC buses were not built by Brown-Boveri. Brown-Boveri built trolleybuses using the GMC TDH bodyshell for Edmonton.

Does the width of the bus itself determine whether or not they have "GM" or "GMC" logos on the front end? I know that the 102-inch wide buses have three inches of space between the logo and an airvent on the passenger side, and the 96-inch wide buses has no space between the logo and the vent.