A security gateway is a security appliance or server which protects your network against spam, viruses, malware, phishing, spyware and more. Ideally you should also have individual security software on each computer as well.
Traditionally a security gateway is an appliance, costing a few hundred dollars or more that you connect to your network. Companies such as SonicWall, Cisco and WatchGurard make these appliances. Here’s a few from CDW.
Untangle has launched a free (open source) security product, which installs on any Windows PC and makes it a security gateway with 14 different and integrated security applications all wrapped into one.
If you’re not technically inclined, I wouldn’t suggest you install your own security gateway. But if you’re a technology consultant or in-house IT person, Untangle’s solution is economical (free) and robust.