Suppose you pay $5 every month for 100GB and you fill it with stuff. What happens if you don't pay after a while? Will Google hold your stuff hostage until you pay or will they erase it?
From Google's purchase policies and conditions (emphasis mine):
If the auto-renewal fails, a seven-day grace period will be added to your subscription so that you're able to update your payment information. During this grace period, you won't lose access to anything in your Google account or to your current storage plan. The storage plans of Gmail users will be extended for 30 days after auto-renewal fails. Gmail users still have seven days to update their payment information for their current plan, but 30 days to free up space in their account or purchase a new plan.
If your storage plan is canceled or expires
If you cancel your storage plan or when your storage plan expires, your storage limits will reset to the free levels for each product at the end of your billing cycle. Everything in Google Drive, Google Photos, and Gmail will still be accessible, but you won't be able to create or add anything new over the free storage limit. If you reach or exceed the free storage limit:
- Google Drive: You won't be able to sync or upload new files, and syncing between your Google Drive folder and My Drive will stop. You'll still be able to create Google documents since they don't take up any storage space.
- Google Photos: You'll only be able to upload new photos and videos stored at High quality.
- Gmail: Incoming messages to your account will be returned to the sender.
Per the Google Drive FAQ:
If you exceed your quota limit, you'll receive warnings in each product and you'll need to correct the issue as soon as you can. Otherwise, you’ll be unable to upload additional items to your Drive or photos to Google+, and, after a period of time, incoming messages to your Gmail account will be returned to the sender and you won’t be able to send new messages.
Also, your credit card on file is automatically charged to pay for storage and you cannot downgrade your storage if you have more than the amount you are downgrading to. Essentially, they do not want you to exceed your storage limit without paying, and if you do you will be blocked from using Google services until the issue is addressed.