Embedding on a Google Site
Embedding into a Google Site is also fairly simple. There are two ways you can do this:
1. Copy/Paste iFrame directly into the HTML
Step 1: Show HTML source for your page
When you are editing your Google site, to the far right of the control menu is an "html" button for toggling directly into the html code.
Step 2: Edit HTML Pop-up
Now you should see an "Edit HTML" pop-up for entering your embed code.
Step 3: Go to the Embed Codes dialog and Copy Code
Step 4: Paste the embed code into the Edit HTML pop-up.
Step 5: Update and Save
You should now be able to see a preview of your site!
Warning: your code will now be unrecognizable!!
If you go back to edit your site, Google will have messed with your code, as you can see above. So, sometimes it's better to use the native Gadget UI instead of trying to edit the HTML (see below).