You will enter your own Google Places API key(s). To get a key, you'll need a valid gmail account and a unique payment method added to the account.
Each grid point represents an API call and you can see places API billing here: https://developers.google.com/places/web-service/usage-and-billing
Google lists the pricing schedule and monthly free credit usage here: https://cloud.google.com/maps-platform/pricing/sheet/
So $200 in Places API credit translates to 5000 'grid point' API calls per month before you start paying 0.032 USD per grid point call. 3x3 grid = 9 grid points and 13x13 = 169 so you can do the math on how many API keys you will need given your usage.
A Google Places API key gets $200/mo in credit when it is enabled under a unique Google/Gmail account and has a unique payment method attached. You can give instructions to your clients and request API keys from them to greatly increase the amount of calls you can make over a given month for free.
At any point in time you can view your usage in your Google Places API console.
Please note that GeoGrid billing applies to your GeoGrid archive searches (enabled from your location's keyword table) and those are by default 3x3 grids.
We are working on adding a locking and alerting mechanism so you won't accidentally go over your free monthly credits.