You can use an iFrame gadget to embed another web page in your Google Sites web page.
(But see Display HTTP iframes)
On the page where you wish to embed the external web page:
Edit Page -> Insert -> More Gadgets -> Featured ... and select the 'Include gadget (iframe);
Edit the 'Properties' of the gadget and include the URL to the page that you'd like to embed into your Google Sites web page.
The subpages below this one show some examples.