Stripe payment gateway doesn’t allow you to do any normal transaction that is less than a $1.
So if you want to offer free downloads, then using a download monitoring type plugin can be helpful. The simple download monitor has the following addon that lets you offer the download in exchange of the email that gets added to your mailchimp list:
In the future, we will look at adding an option that can maybe bypass the stripe checkout if you want to sell a product for free.