To answer your question, you have to go to the replay theater and once you select the clip you want, click on export to save data. Once you do that, you need to quit the game and go to the game section of the PS3 bar and find the icon that says game saved data (or something like that). In it should be a Gran Turismo Data Save and a Gran Turismo Replay 001 saves, and so on depending on how many replays you've exported.
At this point, you need to have your flashdrive connected to the PS3 and what you want to is is press menu (triangle) on the replay file you want to export and select the "copy" option. Then select your flashdrive from the list and it should make a copy onto your flashdrive.
However, if you were planning on putting this on your computer, it will not work. The only thing you can gain are two Gran Turismo images and the rest being the video files, you cannot play on the computer because they require the games files in order to be converted to playable video.
The only real reason to do this is so that you can transfer the files to a friend to put on his/her PS3 so that they may watch your replays.
