I thought that Gb Lan had (marginally) lower latency, as compared to 100 Mbit? That could still prove useful, for LAN-based FPS games.
If you already have an installed base of 10/100 networking gear, then it wouldn't improve much for only games, although Gb is much faster for file-sharing, of course, and media-streaming (think, watching DVD movies over your network!). But for new machines and networking gear, most new mobos come with integrated Gb, and prices for Gb-capable NICs aren't very much more than good-quality 10/100 NICs. So for new gear, I do recommend Gb, at least for the client NICs. When prices come down on Gbit SOHO switches, then you could upgrade cheaply, and have a killer local LAN for file-sharing/media-streaming.
(One small thing that *would* improve, in terms of games on Gb LAN, would be level loading times on games that download the level from the server, like the UT series of games. I hate playing a level at a LAN party that I don't already have cached on my machine, and it takes 2 minutes to download off of the server - by the time it has downloaded to my machine, that match/level is usually just about over.)