To use BGP routing on an AWS or GCP VPN connection, the tunnel interface needs to have its IP address assigned as a /32 and then the remote IP specified:

config system interface
    edit "GCP"
        set vdom "root"
        set ip 169.254.0.2 255.255.255.255
        set type tunnel
        set remote-ip 169.254.0.1 255.255.255.255
        set interface "wan1"
    next
end

BGP can be configured under the GUI in Network -> BGP in most cases, but the CLI has additional options. Here’s an example config for a peer 169.254.0.1 with ASN 64512, announcing the 192.168.1.0/24 prefix.

config router bgp
    set as 65000
    set router-id 192.168.1.254
    set keepalive-timer 10
    set holdtime-timer 30
    set scan-time 15
    config neighbor
       edit "169.254.0.1"
           set remote-as 64512
       next
    end
    config network
        edit 1
            set prefix 192.168.1.0 255.255.255.0
        next
    end