Jenn, check out itDuzzit, I think it's what you need: 
https://www.itduzzit.com/connect/marketo-to-google-docs
Looks like you can send new leads to a Google Sheet and hopefully you could put some constraints on that to see only leads who had filled out x form. 
If not the webhook scenario is your friend. 
Let us know if you get it working !
Justin Norris | 
Perkuto