GCP cloud router is not accepting default route advertised by a on premise router

[Pseudonym10]

We have set up a VPN tunnel from our premise datacenter to a Google Cloud project. The BGP session on our on premise router, and on a Google Cloud router is successfully established.
The traffic flow is to route all egress traffic from Google Cloud instances to the on premise network. However, the default route is not accepted by Cloud router and not added to the routes table.