This is lifted from the Toyota Nation Avalon Forum, I don't know if it may or may not work on the GS, posted by Preskev
http://www.toyotanation.com/forum/showthre...7377&page=3
******FOUND THE SOLUTION TO DISABLE THE SEAT BELT CHIME*****************
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Well, no wiring mod is required. Here is the procedure:
1. Set the LCD on the combination meter to the ODO trip.
2. Press and hold the TRIP meter reset knob for ten seconds or more within six seconds of turning the ignition switch to the ON position. Fastening the driver's seat belt while holding the knob cancels the buzzer reminder. (You may have to hit the reset meter knob to cycle through b-ON to b-OFF)
3. When this occurs, the LCD displays "b-OFF". The buzzer reminder can be switched between ON and OFF each time the TRIP meter knob is pressed. The LCD displays "b-ON" or "b-OFF" according to the buzzer reminder ON/OFF status. The display changes to the ODO or trip meter if the knob is not pressed in 10 seconds.
4) Performing steps 1) and 2) again when the buzzer reminder is off exits cancel mode.
I was able to disable the seat buzzer and took a drive. IT WORKs! I am so relieved.