What I did was I saved the GP60M as a CDP file , then loaded it into an older version of Trainz - in this case I saved the asset from TRS2019 and loaded it into TS2010 content manager since that game doesn't have the GP60M built in. While in content manager I cloned the asset, then created a new version of the locomotive (to avoid overwriting the built-in GP60M in TRS2019), saved the newly created GP60M as a CDP file, and loaded that CDP file into TRS2019's content manager. Doing that ended up letting me edit the asset and I am able to change the dependencies in the locomotive such as the horn and enginesound.
Hope this was descriptive enough, and easy to understand.