LocalBasemapsSource

// in this example, the portal item is a webmap
let basemap = new Basemap({
 portalItem: {
   id: "867895a71a1840399476fc717e76bb43" // Mid-Century Map
 }
});
let basemapGallery = new BasemapGallery({
 source: new LocalBasemapsSource({
   basemaps: [
     Basemap.fromId("topo-vector"), // create a basemap from a well known id
     basemap
   ]
  }),
 view: view
});
view.ui.add(basemapGallery, 'top-right');