How to use Ubuntu PPTPD as Gateway?

I just set up Poptop on our Ubuntu 10.04 server so we could use it as a VPN.

I can connect to the LAN through PPTP but I can't use it a gateway (I needed to uncheck the "use default gateway on remote network" option on the Network Adapter properties in Windows.

We don't actually need (nor want) to to route all internet but there is certain stuff that is restricted to our office network's IP address such as SFTP access to hosted web servers.

Most of my settings is default except that I'm using the same subnet as our LAN. I've also enabled ip forwarding in sysctl.

How can I use it as default gateway?