When using 'Always synchronize with the Basemap Gallery setting of the organization' option in Basemap Gallery widget, services do not show up even though spatial reference is set to 102100.
Instead of using 'Always synchronize with the Basemap Gallery setting of the organization' option, use the 'Configure custom basemaps' option and manually add each service as 'Create new' > OK. The widget now supports and use manually added services.