The iFrame code from Google is deleted automatically by TinyMCE (The editor). But it is javascript, so here is an easy hack.
Turn off javascript in your web browser when you edit. If you are already on the paragraph editing page, you'll have to refresh.
Then paste the Google code and submit.
You'll have to turn javascript back on to view the Google Calendar.
NOTE: If you ever edit the page with javascript on TinyMCE will erase it.

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This was a great question. In the future I'll tweak the Tiny MCE editor to allow iFrames.