How does a Linux server allocate multiple VPNs at the same time and share them as gateways?

How to allocate multiple VPNs to Linux servers and share them as gateways-Linux Enterprise applications-Linux server applications? For more information, see the following. Hardware:
Gateway 1: China Unicom ADSL route, wanl port connected to the Internet, Lan1 connected to the switch, and the Intranet ip address of the route: 192.168.20.1
Gateway 2: it is a China Telecom ADSL route. The wanl port is connected to the Internet, Lan1 is connected to the switch, Lan2 is connected to the VPN gateway, and the Intranet ip address of the route is 192.168.20.1.
VPN Gateway: it is a VPN dial-up server with two NICs. One Nic is idle and one Nic is connected to Gateway 2 (public network of China Telecom and serves as the Intranet gateway). The IP addresses are 192.168.20.20, 255.255.255.0, 192.168.20.2.
Number of clients
Others:
Three VPN accounts (PPTP): US, UK, and Japan
Expected results:
The VPN gateway allocates three VPN accounts at the same time and shares them with the client as the gateway.
For example:
The client uses the gateway 192.168.255.20 to access the U.S. VPN.
Use 192.168.20.21 to access the British VPN
Use 192.168.20.22 to access the Japanese VPN
Can the above effects be achieved without adding a VPN gateway (you can add a NIC,
It is best to use Linux to achieve this effect.