Internal communication in GCP through Fortigate

Dears,

Is there is away to force all the internal-vpc subnets to be through the fortigate,

the setup is like this:
external-vpc with a external-subnet and there is a default routing 0.0.0.0/0 pointing to the internet gateway.

internal-vpc with two sebnets backend-subnet and backend-subnet pointing to fortingate internal interface ip address.
Is there is away to force backend-subnet and backend-subnet to communcation through Fortigate. since they have a default route to commnicate directly right now.

Kind Regards.