if you are using bridging on a machine to allow another network (say network B) to access a gateway on network A, then you wouldn't need WinGate as well on this bridging machine, as the machines on network B should simply be able to use the gateway address on the network A.
In order for this to work, all the machines on network A and B will need ip addresses in the same
logical subnet. If they are in different logical subnets, you would need routing between the two subnets. XP should do this for you as well without requiring WinGate. You would need to add a route on your main gateway machine though so that it knows how to reach network B by going via the address of the machine you are currently running bridging on.
You should read the paper on routing at
http://www.qbik.com/files/routing_paper_letter.pdf
Adrien