It may not be, but I guess it depends on what you have handy. But most of the info will still be useful no matter what hardware you use :)

I did this with an old Mac Mini that was acting mostly as a media server. My router wasn't compatible with TomatoUSB, so I went with what I had. The toolchain was a bit different since OSX uses the BSD networking toolchain rather than the Linux iptables & such.

Edit: s/GNU/Linux

IPTables isn't part of GNU. It was developed by the Netfilter team. Indeed, very few of the networking utilites that are common in Linux distributions are part of the GNU project.