If you correctly set up the SLIP link you shouldn't have needed to install Wingate. Windows 98SE onwards includes "Internet Connection Sharing" which will happily forward packets from your internet connection to the SLIP line.
Caveats; it refuses to work on any other configuration than plonking itself at 192.168.0.1.
Without knowing what your current network is beyond connecting an Amiga to a PC, it's a little hard to say if that would be adequate? I assume though since you're going so far as using a serial line, that your "network" is entirely absent
