Web AppBuilder for ArcGIS is unable to display custom basemaps from the Basemap Gallery widget if the custom basemaps are created using the Open Geospatial Consortium, Inc. (OGC) Web Map Service (WMS) map services.
Duplicate of BUG-000086388: This is a known limit. For more details about what are supported and unsupported basemaps, please access http://enterprise.arcgis.com/en/portal/latest/use/widget-basemap.htm.