Actually, the sitemap.ShowSections appconfig is for yoursite.com/sitemap2.aspx. However, if you really want that to trigger in googleindex.aspx you can follow the code below to edit the googleindex.aspx.cs.
from
Code:
Response.Write(AppLogic.SectionEntityHelper.GetEntityGoogleSiteMap(0, Localization.GetWebConfigLocale(), true, true));
change it to
Code:
if (AppLogic.AppConfigBool("SiteMap.ShowSections"))
{
Response.Write(AppLogic.SectionEntityHelper.GetEntityGoogleSiteMap(0, Localization.GetWebConfigLocale(), true, true));
}
Note: I'm not sure if this is recommended, so you have to refer to google sitemap documentation.