If you haven't installed, please install from Atlassian Marketplace here: https://marketplace.atlassian.com/apps/1221472
Once installed, open macro browser and select HTML/Iframe Macro.
Here are some example use cases:
First, create form in Google Forms and copy iframe code which you can find by clicking send or share button.
Open HTML/Iframe Macro macro in Confluence and enter the copied Iframe code in textbox.
Save and don’t forget to publish it. Your Google form will be beautifully rendered in Confluence.
That’s it. Now users can fill form without leaving Confluence. Their responses will be saved in Google Form.