Hmm I haven't heard of GT compression. Mouse, it's ALWAYS a 2 stage process with every video format except raw avi if you wanna be specific.
Except what's the point of saying that? All players compress and play on-the-fly anyway.
Gabrielle's advice is good: I use G-spot myself and then getting a filter pack should sort you out. I haven't tried the one she suggests, I use Tsunami filter (codec)pack which has basically any you could ever want. But after you identify with Gspot, you should be able to search for that particular codec anyway.
A final bit of advice: get VirtualDub. This is a brilliant program that can read all video formats for which a codec is installed and can convert between them. It can also uncompress and recompress audio, change framerates etc. If you have a video file that is too big you can easily compress it with VirtualDub...you can set the audio and video bit rate etc.