Well, when diesels first came into appearance, it was all F-units. Then, EMd decided to create the geeps.
The GP7 wasn't made until 1949, so before that, I guess you'd have industrial switchers made by smaller companies.
In addition to those small industrial switchers, Alco was building high hood switchers like the HH600s and HH660s between 1931 and 1940, so you'd see those running around. Atlas makes a model of the HH600/660s in HO scale.
Fairbanks Morse created their H15-44s in 1947.