The D&RGW had only one 36 inch gauge diesel, #50 (they did borrow two Army ones for a while in the '50s). The GP9 you saw was one of their standard gauge machines.
50 was based in Durango. In later years Alamosa was one of the few (I want to say 3) meeting points of Standard and Narrow gauge on the D&RGW. That is most definately a standard gauge GP9, possibly assigned to the Creede branch.
Alamosa's joining of Std. and NG makes for some interesting photos.
All via the Denver Public Library: K-37 491 posed with F-81 (2-10-2) 1403.