Ok just as a pretext my issue was that i wanted to join some LAN joint operations servers but my pc connects to the internet via an ADSL card that uses PPPOA. GIT manipulates ethernet frames which caused a problem as i dont have an ethernet home network or etherent card.
I dont know whether this solution has been posted before nor whether it will work for all games but here goes.
What you want to do is install a loopback device which will act as a virtual ethernet card if you will, in windows xp you can do this by running the add new hardware configuration, going to advanced and selecting the microsoft loopback device driver (may not be available in previous windows OS's).
Once installed you will want to give this driver a private IP address (e.g. 192.168.1.1) subnet mask and default gateway. Make the default gateway your external IP address as assigned by your ISP.
Then go into the GIT advanced configuration and select alter source IP (bottom right corner) make the From Hostname (internal): the IP address of your loopback device and the To Hostname (external): the IP address assigned to you by your ISP.
That should have the effect of creating an ethernet LAN game frame on the loopback device, the packet(s) Source address being manipulated and being sent out your PPP device.
The above is a bit sketchy and ill try to put together some how-to screenies if people want them. Sorry if this solution has been discused before but i couldnt find any post.