Is it possible to connect a plurality of VPN gateway to one of the Managed Firewall?
Yes, it it possible.
However, depending on the specification of the VPN gateway, when multiple VPN gateways are connected to the same logical network, it is impossible to communicate because the MAC addresses are duplicated.
(Excerpt) The problem of this inappropriate configuration was solved through maintenance works on January 16, 2018, and thus the configuration is currently usable. Managed Firewall and Managed UTM generated before January 16, 2018 are also usable. However, if multiple VPN gateways are connected with the same logical network, duplication of MAC addresses results in communication failure due to the specifications of VPN gateways.