I have configured a network map in Virtual Box. I have one VM that acts as a host on a network. I want this host to access a private network that is located behind a gateway, through a tunnel. The private network prefix is 10.0.20.0/24 and it is connected to a gateway 10.0.20.1. The gateway is connected to another network through another interface 192.168.20.5. The VPN server will be installed on the gateway. I have created a tunnel and assigned it an IP address on the gateway, then I did the same on the host thus connecting to the server. I get that the client is connected to the server and the server to the client.The problem is that I can not ping from the outside host to the private network. I think the problem is my routing table. On the external host, I set default gateway as the tunnel interface. And on the gateway, I add the net where the provate network is and set the gateway as 10.0.20.1Is that correct? 
Create tunnel from Host to Gateway
vpn;gateway