Here are some alternatives rather than solutions:
has a lot of ways to make your linux box look like windows, but I can't really find anything that will act like the system tray. If you are looking to just not have it be in the taskbar when you open it, you can use something like kstart. The thing is, that the page I referred you to, and the kstart, are all kde things, not gnome. I will keep on looking for thing for gnome for you though. But for kstart, do a man 'kstart' to see how, but it will be something like:
Open a terminal and type:
kstart --window gaim --alldesktops --skiptaskbar /usr/bin/gaim
But the location of gaim may be different on your system. That's just to make it so it doesn't go into the "taskbar" and present an icon there, it doesn't add anything to any "system tray" type thing for when you minimize it.