I need to add an option to the basemap gallery "No Basemap" this is to hide the basemap, but unable to to it, even tried to add a blank map as basemap. but I am getting an error "The basemap you are adding has a different spatial reference from the current map." How do I hide the basemap/ add a no basemap
You could do this by adding any of the preset basemaps to your web map then changing the transparancy of that basemap layer to 100% transparent. Then you can rename the basemap to "Blank" or "No Basemap" see image below.
Let me know if this helps!