Just a thought from my perspective. I would agree that the ash pit would be a better location for ease and use. The ash pit is also one place every locomotive visits and is a more likely spot for a crew to participate in this process.
The picture at Salida shows these boxes on the ground and stacked on the “pallet” to keep them off the ground. Also it seems a little like over kill on the size of the Gunnison out riggers.
However, the picture of Salida appears very small untidy or even cluttered. I wonder if this at all creates extra problems around this work area? On the turntable, this placement would keep them out of the mud as well as away from the work area around the pit (or even a non tripping issue). Also, the out riggers look like they provide a rudimentary shelter from the elements. Lastly, being on the turntable means you could use a space that is largely not being used (i.e. the open space on either side of the he table).
So I now wonder, was the Salida yard cluttered in some areas vs Gunnison being more tidy, like someone in charge might have been OCD?
It is a fascinating issue that without it written in a viable sourse or a first hand account, there may not be a definitive answer. But I have enjoyed the speculation and the bonus of seeing the various postings about grip boxes.
Phil