Brent-
Atlas uses Lionel's TMCC products, so I would recommend using one of Lionel's TMCC diesel wiring diagrams as a starting point, such as one for the F-3, or the SD80MAC. The R2LC, RailSounds and RailSounds power boards are all the same, and the motherboard should be very similar. Since there is no smoke unit on the GP15, you can ignore the wiring for the smoke unit. As an aside, these diagrams are also helpful for working on late model K-Line diesels equipped with TMCC and RailSounds.
If the wiring does not match the diagrams above, then it is likely Atlas used the Cruise Commander product from The Electric Railroad Company. The manual for the Cruise Commander is available here.
Hope this helps!
-John