Ideally what you'll want to do is create a page in your content tree. Many already have a /Sitemap page that is a visual for visitors, so create something like /Global/Sitemap. On that page, place the XML Sitemap webpart. In there you can set the properties you'd like, like which doc types to select, where in the node to start, a transformation, etc. This webpart offers more properties than the basic /CMSPages/GoogelSitemap.aspx.
Then after you have configured that webpart go into the Settings application and find URLs & SEO. In there set the Google Sitemap path property to the /Global/Sitemap page (or whatever you called it) and it will automatically serve up the sitemap on that page when someone requests /googlesitemap.xml.