This is not a perfect answer - but an easy one.
Create a simple "Click Through" splash page, asking users to fill out the Google Sheet. Then go into the Splash Page configuration (down the bottom) and use the option to redirect the user to a custom splash page. Put in your Google Sheets URL.
Note that the user is actually allowed by this stage so they could just close the sheet and continue on, but it will look like it is part of the experience so they will probably just fill it out.