更新：
2018 年 11 月开始，G Suite 以外也支持独立域名了，
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https://support.google.com/sites/answer/9068867?hl=enUse a custom URL for your site
You can use your custom URL with a site on new Google Sites by creating a web address mapping.
Step 1: Check that you own your URL
Go to
google.com/webmasters/verification.
Look under "Properties."
If you don ’ t see your URL, click Add a property.
If you see your URL, move to Step 2.
Enter your URL.
Click Continue.
Follow the onscreen steps.
Click Verify.
Note: To use custom URLs, you must be the owner of a site on Google Sites and it must be published. Learn how to publish your Google site.
Step 2: Update your URL on your host site
For your custom URL to map to your site on new Google Sites, you need to create a CNAME entry with your domain host, like Google Domains, GoDaddy, or InMotion.
Note: These steps may be different depending on your host site. For more help, go to your domain host ’ s help site.
Go to your domain host.
Create a new CNAME entry.
Enter the subdomain name, for example, the "www" of
www.YourURL.com.
Type in the host name
ghs.googlehosted.com. This will point your URL to your Google site.
Step 3: Assign your custom URL
On a computer, open a site in new Google Sites.
At the top, click More More and then Custom URLs.
Enter your custom URL.
Click Assign.
Note: You can add up to 5 URLs to your Google site.
...
You ’ ve reached the maximum number of URLs for this domain: There is a limit of 20 mappings made per week for your URL.
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