Logan's way will work too, but instead of placing ip addresses into each server/service that you want to allow access to, you could use User Assumptions instead. i.e.
You could use the "Assume users by ip address" option, and then modify the System Policies to state that the "Everyone" group must have an authentication level of "User may be assumed". - This will allow all your users to be authenticated when connecting to WinGate. You could then navigate to the WWW Proxy Service --> Policies, change the "Default Rights" menu to "Must also be granted", and then add in the users/groups who can access the WWW Proxy Server.
Assume by ip address:
*The "System Policy" option is below the "Assumed Users" option shown in the red box.
**You may also want to turn on Intercepts for port 80 in the WWW Proxy Server --> Sessions, so that other connection methods to a remote server on port 80 are pushed up through the WWW Proxy Server so your polices can be applied.
***The NAT connection method is controlled by the Extended Networking Service (ENS) policies. The WinGate Internet Client (WGIC) connection method is controlled by the Winsock Redirector Services (WRS) policies.