I own and drive daily a 2003 gs 300 with 152000 mi and have never changed the timing belt is this a necessity??

I would have the timing belt replaced asap, as it should have been replaced at 90,000 mi. Hope this is what you needed to know. Good luck.

YES, they are due at around 90-105k miles, most change them out sooner. There are other parts that need to be replaced as well once you are going to do it ie. Water Pump, Timing Belt Tensioner, Idler Pulleys. Today you can buy kits that those parts come in. Would highly recomment replaceing all those parts as you dont want to have to go back in and replace them later if they fail.

Check your owners manual/maintenance interval, but if I were you I would at least do an inspection. If the belt breaks or even slips a tooth, you will be stranded. The belt could possibly last another 150K, but I wouldn't want to risk trying. If you need more advice, please feel free to reply.