Add those lines to the web.xml:
  <servlet>
    <servlet-name>GisViewerService</servlet-name>
    <servlet-class>org.gcube.portlets.user.gisviewer.server.DefaultGisViewerServletImpl</servlet-class>
  </servlet>
  
  <servlet-mapping>
    <servlet-name>GisViewerService</servlet-name>
    <url-pattern>/<YOUR APPLICATION NAME>/ViewerService</url-pattern>
  </servlet-mapping> 
