Google has retired support for directly submitting geositemaps in 2012. You can continue to submit your Geo content though using the standard sitemaps format.
Sitemap for mapsmarker.com: https://www.mapsmarker.com/sitemap.xml – geositemap is added to https://www.mapsmarker.com/sitemap-misc.xml (via XML Sitemaps plugin for example)
Please use one of the following sitemap plugins, which will automatically add your geositemap to your sitemap: