First of all we are going to use a nice little tool called Google sites liberation (GSL). If you don't have time to read what is all about this tool I will quote from the User's Guide:

The Sites Liberation import/export tool uses the Sites GData API to allow users to export an entire Google Site as static html pages to a directory on their hard drive.

1. Download GSL locally.
[codesyntax lang="bash"]

wget -c http://google-sites-liberation.googlecode.com/files/google-sites-liberation-1.0.4.jar

[/codesyntax]

2. Execute GSL
[codesyntax lang="bash"]

java -jar google-sites-liberation-1.0.4.jar

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3. To export google sites data.
If you do use google apps then you will have to fill the fields as follows:
Host: sites.google.com
Domain: test.org
Webspace: webspace

Username: user
Password: password

If you don't use google apps then you will have to fill the fields as follows:
Host: sites.google.com
Domain:
Webspace: webspace

Username: user@gmail.com
Password: password

Where can you find the webspace? The webspace is usually the latest word in the link. Just take a look at the URL in your browser.
https://sites.google.com/site/webspace/
https://sites.google.com/a/test.org/webspace/