Nice to see it has, after too many years, been given a paint job and a cover
I saw it ca 1973 in dismal shape, as were all the locomotives on 'display'

Are we sure of the facts from the interweb??
My thought is that SP told WPB 'GS'. stood for General Service.
As to redesign of the streamlined versions, not so fast.
Take a look at the diagrams. The never streamlined GS-1, and the original Daylight GS-2 and the GS-6 and the WP and the contemporary C of G engines differ only in the smallest details and accessories.
Never heard why SP was denied the bigger engines, but guess WPB figured the GS-1 would fit more places or better suited to 'General Service'. Please note the Overnights were suspended for the duration. I wonder if there is a pic of any GS on a really freight train taken 1942-1946? Tough years for those who would take pix of trains.
No big deal on the skirts as IFAIK the little GSs spent a lot of time in the Oregon tunnels.