I try again with a new post since the previous one has not been understood.
Using in Photoshop 23.0 the "Gradient Map" adjustment layer even with the "Classic" Method is not backward compatible. Does not work as expected.
I need to specify, since the previous post received a wrong reply regarding the "Gradient Fill", that I'm talking about the "Gradient Map" (not the Gradient Fill!) adjustment layer correctly setted with "Classic" method.
Even with classic method the Gradient Map will be not read in the previous version of Photoshop.
The only way to make it readable in previous versions is convert the image layer in a smart layer and apply the Gradient Map as a smart filter (from menu "image" -> adjustments -> gradient map), always in Classic Method. In this way the gradient map will be read even in the previous version.
So, please, there is a bug with the Gradient Map adjustment layer in Classic method since should be backward compatible.