Barry Hunter has created his own gadget code for embedding Google Maps into Google Sites. The key to using this code is to have your map already created and to have the KML URL for that map.
Use the Insert button followed by More, and use the Add by URL link. In the URL box, add the following URL:
http://hosting.gmodules.com/ig/gadgets/file/114026893455619160549/embedkmlgadget.xml
and use the Add button.
Now, in the KML or My Maps URL box paste your map's KML URL. Choose 2D (Google Maps) in View Mode and click the OK button.
For full instructions and to see how this code will work within Google Sites, visit Barry Hunter’s Embed a “My Map” in Google Sites.
To learn more tips when it comes to using Google Sites, check out these Google Sites Tips.