GCP VPN – Tunneling traffic through two VPN hops

I have the following topology:

On-prem (EU) -> VPN Classic TUNNEL -> GCP -> VPN Dynamic (BGP) TUNNEL – On-prem (AMER).

And I would like to connect both on-prem sites using the GCP tunnels.

A very friendly colleague told me (in GCP forum) that it is possible to do it using Advertising custom IP ranges. However, not sure if is applicable to VPN between On-prem (EU) and GCP because Classic VPN is in use (not BGP).
Is it possible and any idea how to to it?