Not really a dealer issue because you may want to start thinking about replacing all of them but a dealer will be able to tell you which one is bad and just replace that one if you want. A tire shop should be able to do the same but which shop is equipped to handle that? I don't know of any in my area that can scan and replace just one sensor but I do know that their sensors and programming costs are much cheaper. To get a sensor replaced at a tire shop in my area is around $80 each. For one to be done at a dealer, I think is around $350. Still not sure which sensor will last longer since the technology is relatively new. Only time will tell. My advice would be to price quote them first and then make your decision.
Now that I'm sitting here re-reading my response, I have realized that I have been kind of rambling so i will leave it at that. Hope this helps and let me know how it goes

 

Here's an update. I finally made time to stop at another tire shop, Discount Tire. I've used this location before and they do good work.

What they did is let air out of each tire while holding the scanner to the valve stem. When the pressure got low enough, they sent a signal. Passenger front, no signal. Driver rear, weak signal. Others OK, for now. The kicker is that the light stopped flashing. I've driven about 10 miles now. If it comes back on, I'll post back. Still undecided whether to trade it for a 2013 GX or Sequoia, but now would be the time.

 

Denso is the factory supplier for the TPMS sensors in your GX:
Rockauto has them for $33 also.
You need someone with Toyota/Lexus Techstream software to register the new sensor. I just replaced the one in my Prius myself after the wife drove on a flat tire and ruined the sensor. Hardest part was breaking the bead to slip the new sensor in.

 

Just had Discount tire replace one in my 08 GS. 60 bucks and they were able to register it in the system, no need for a visit to dealer.