The Gateway is a part of the VNET and can be used by every VM in the VNET if the NSGs and if used the Routing are set wright. If on the other hand, the Firewall and Routing rules in the on-premise LAN is set wright the VM in Azure can of Course reach multiple SQL instances on prem. I would be considering the latency between Cloud on-premise for the data Access... best Niels here is a good architecture overview about Gateway and Routing https://docs.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-howto-site-to-site-resource-manager-p...
