Get the code required for your gadget.
Start with a plain text editor (Notepad or Notetab. If you use a document processor like Microsoft Word it may embed hidden characters that make the file unusable unless you can save it in plain text mode.

<?xml version="1.0" encoding="UTF-8" ?>  
  <ModulePrefs title="hello world example" />  
  <Content type="html"> 
       Hello, world! 
  1. Change the title hello world example with a name you want for your gadget.
  2. Replace the Hello, world! text with the code you got in the first step.
  3. Save the file with a new filename and a .xml filetype. like "mygadget.xml".
  4. Upload it to a file cabinet page in your site. Do not upload it to Google Drive as that does not work.
  5. Right-click on the download link and copy the URL.
  6. In the page where you want to embed it use Insert, More gadgets, Add gadget by URL. 
  7. Insert the URL you captured in step 5.
  8. Set the width and height as appropriate.
  9. Save everything.
See the Facebook page and the Test page for examples.

Or you can download a gadget from the Gadgets page on this site, edit it as you wish and upload it to a file cabinet page on your own site.

Then right-click on the download link and capture the URL.

Now edit the page where you want the gadget to appear, use Insert, More gadgets, Add gadget by URL and paste in the link.

NOTE: Many gadgets, notably Facebook ones and others,  will not work on a private site!!!  Sites will have to be "Public on the Web". If your gadget gives an error you might want to make sure your site is shared publicly.