As reported in the Google Apps Issue Tracker (star the issue in the tracker to be kept updated)
What steps will reproduce the problem?
1.write a script in a spreadsheet and publish as a service
2.use 'insert apps script gadget' in a site
3.paste the URL of your Apps Script already published as a service
What is the expected output? What do you see instead?
gadget should appear in Google Site but instead get error message 'Unable to insert object due to invalid URL'