I recently downloaded Web AppBuilder 2.4, Developer’s Edition and built a quick & dirty app. For some reason, it worked fine in Web AppBuilder but once I deployed it, no basemaps would show in the Basemap Gallery widget, it would just spin, spin, spin. Digging around, I found some old posts where users were experiencing similar problems but none of the solution resulted in a great solution. I was able to get it to work by listing the basemaps individually in the config_BasemapGallery.