A lot (LOL). The GP15 is a SW(MP)15 in a road unit body.
A GP9R
A GP15(A UP GP15-1)
A GP9 always have radiator fans (2 on the GTW unit) where the UPY 674 has a "tunnel motor" radiator section. PLUS...GP9's are only 1750 hp(GTW GP9R's are 1800 IIRC) where the GP15's are only 1500.
No sir, the GTW never had GP15's. When the GTW was looking for power to replaces their switchers, they also needed something that could be used on the road, the GTW probobly would have gotten some GP15's if EMD had still been building them. Thus, the GP9R program was started.
Dave C