No, this is not a billable event, you can configure the gateway anyway it will allow. However, many people have reported problems with setting up port forwarding on this device, it's one of many problems with it. Some can get it to work, others can't. I HIGHLY recommend you have the SMC placed in bridge mode by the gateway support folks at 800-363-2416, get your own router, and put your port forwarding configuration there.
Do be aware that running a server on your Comcast residential service is subject to some restrictions. The server(s) can only be for limited personal use, no commercial or wide spread public usage, keep it clean/legal, etc. If Comcast finds anything commerical (ie. you use it to make money), or they notice lots of traffic or content of a questionable nature, they will likely take some action up to and including suspending your account. Typical uses include file/photo/video sharing with family and friends, running a game server to play Counterstrike with your buds, etc.
