You didn't specify which scale but mentioning Atlas and Walthers makes me think HO.
TrainWorld has Proto-2000 GP7s and GP9s in various road names starting at $39.
http://www.trainworld.com/lifelike/lifelike_proto.htmhas
I would say other than the F units, the GP7-9 are the most over produced diesel locomotive model in HO scale. Athearn, Atlas, MRC, Lifelike Proto-2000, and Front Range all come to mind off the top of my head. For a while one couldn't open a magazine without Proto-2000 or Atlas publishing another "run" with new road names for them. For the time line specified you could also include GP18 and GP20s. Once again Proto-2000 has made multiple runs of both those. And GP30s came out in 1961.
I would stick with the Proto and Atlas Geeps for HO.
Only the GP30 Demonstrator was built in 1961, regular GP30 production started in 4/62 with an order for 20 Reading units.