Although a VNET can only have 1 VPN Gateway, you can connect multiple devices via P2S VPN to that Gateway.
To answer your question of what to do, you have 2 main options:
1) Connect clients to the P2S VPN in your existing VNET. This is the easiest option.
2) have clients connect to other VNETs via P2S, and peer those VNETs with your existing VNET. You will not be able to have a VPN Gateway in your existing VNET in this scenario. Also, clients connected to different virtual networks will not be able to directly communicate with each other, so this will have better security.
You might also be able to find a 3rd party NVA that can accomplish part of this.