Hello,
Have you configured a traffic-selector under 'edit security ipsec vpn <name>' hierarchy for the existing subnet?
If yes, you will just go ahead and create a new traffic-selector there that now calls the new subnet behind
google cloud as remote subnet & your subnet as local subnet. For latest Junos, when VPN comes up, it should automatically install route through st0.3 for remote subnet.
If you do not have any traffic-selector or proxy-id under 'edit security ipsec vpn <name>' & using route-based VPN, then I guess peer also supports 0.0.0.0 proxy-id and hence only thing you need to add is route to remote subnet pointing to st0.3 interface.
Regards,
Rushi