This is not realy a perl question.

An internet search for GTP turns up GPRS_Tunnelling_Protocol Is that what you mean?

If you are looking to tunnel internet packets through another networking protocol, then you need to setup a VPN. If you want to limit the traffic that can go through that VPN, then you need to put a firewall, and possibly a proxy in front of it. So to limit the websites that your users can access, you could setup the firewall so that it will only accept web connections from the proxy, and setup the proxy so that only a white list of sites are allowed.