Lloyd,
OK that seems to be a bug. Here is a workaround if you are going to use a basemap like gray that has a reference layer to initialize the map:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no"/>
<title>Basemap gallery</title>
<link rel="stylesheet" href="https://js.arcgis.com/3.18/dijit/themes/claro/claro.css">
<link rel="stylesheet" href="https://js.arcgis.com/3.18/esri/css/esri.css">
<style>
html, body { height: 100%; width: 100%; margin: 0; padding: 0; }
#map{
padding:0;
}
</style>
<script src="https://js.arcgis.com/3.18/"></script>
<script>
var map, refLayerId;
require([
"esri/map", "esri/dijit/BasemapGallery", "esri/arcgis/utils",
"dojo/parser",
"dijit/layout/BorderContainer", "dijit/layout/ContentPane", "dijit/TitlePane",
"dojo/domReady!"
], function(
Map, BasemapGallery, arcgisUtils,
parser
) {
parser.parse();
map = new Map("map", {
basemap: "gray",
center: [-105.255, 40.022],
zoom: 13
});
var basemapGallery = new BasemapGallery({
showArcGISBasemaps: true,
map: map
}, "basemapGallery");
basemapGallery.startup();
basemapGallery.on("selection-change",function(){
var layer = map.getLayer(refLayerId);
if(layer){
map.removeLayer(layer);
}
});
basemapGallery.on("error", function(msg) {
console.log("basemap gallery error: ", msg);
});
basemapGallery.on("load", function(msg) {
for (b=0;b<map.basemapLayerIds.length;b++){
var lyr = map.getLayer(map.basemapLayerIds[b]);
if(lyr._isRefLayer){
refLayerId = lyr.id;
}
}
});
});
</script>
</head>
<body class="claro">
<div data-dojo-type="dijit/layout/BorderContainer"
data-dojo-props="design:'headline', gutters:false"
style="width:100%;height:100%;margin:0;">
<div id="map"
data-dojo-type="dijit/layout/ContentPane"
data-dojo-props="region:'center'"
style="padding:0;">
<div style="position:absolute; right:20px; top:10px; z-Index:999;">
<div data-dojo-type="dijit/TitlePane"
data-dojo-props="title:'Switch Basemap', closable:false, open:false">
<div data-dojo-type="dijit/layout/ContentPane" style="width:380px; height:280px; overflow:auto;">
<div id="basemapGallery"></div>
</div>
</div>
</div>
</div>
</div>
</body>
</html>
