It was highly asked question from Google Sites users that "How to create horizontal navigation in Google site?". Now Google Sites has added this nice feature. You need to understand that horizontal navigation is just navigation bar, not the drop down navigation menu. The navigation bar will show on top
of each page of your site.
Secondly, users have that multiple options like "Boxes" (the default), "Tabs" and "Links" to display it.
Click at "More action" and then click "Manage site".
Click "Site layout" under "Site appearance" and then click "Change site layout".
In the appeared box, Select "Horizontal navigation bar" and click "OK".
No you notice that your site layout page got a more option ""