The account will automatically top up when you don’t have enough money in your PayPal balance to complete a transaction.  You can also manually add money to your account on paypal.com or through the PayPal app.

Need more help?

How do I turn off automatic top up for Google Pay?  
How do I change how much comes out each time my Google Pay account automatically tops up?
Where can I use PayPal with Google Pay?
How can I add PayPal to Google Pay?
How can I withdraw money left over from an Google Pay transaction?
How do I update or reset my Google Pay PIN?
How do I remove PayPal from Google Pay?
How do I make PayPal the default payment method on Google Pay?
Can I have multiple PayPal accounts linked to Google Pay?
What happens if my mobile device is lost or stolen?
How can I get a refund on an Google Pay purchase?
Why do I need to create a PIN for Google Pay?