VPN routing options

Looking for VPN routing solution.
A terribly bad picture below but it points out what I need to accomplish.
Two seperate client VPNs connect to my gateway.
Then I forward the traffic onto a third VPN to our service provider.
Two way communication must pass between Clients and Service provider.

Client 1 CE——-VPN—————\
>>>>>>>>>>>>>>>>>>>>>>>> My Gateway ————VPN——-Service Provider
Client 2 CE ——-VPN————-/

Is there a software solution that does not require an exacutable for the client and service provider?

Is there a Free/open source solution?

can this be done with one IP address for my Gateway?

What options are there in general what do large service providers do to accomplish this?