linux – VPN including IP subnet

I have a company network having network “A”. Inside this network there is a subnetwork “B”: this subnetwork has one Linux-operating machine, a Gateway, having two network interfaces: to “A” and to “B”. I have full root access to Gateway.

Also I have a PC connected to “A”.

I would like to connect a PC to “B” but having an IP and netmask and whatever of that network “B”.

I already established an IPsec VPN and I can access computers from “B” directly from PC by their IP names.

But not vice versa: any computer from “B” doesn’t “see” a service opened on PC, also multicast/broadcast messages from PC do not reach “B”.

Please assist, what should I google for?

The next step is to set up a Wifi Access Point on PC to provide “B” access over WiFi to another devices.