The only option you have for A/M head units in a GX with nav, is pulling the dash, replacing the dash harness with a non-nav harness, buying the non nav HVAC controls, and going from there. It may also need a BCM among other things, depending on what communicates with what. It's not going to be cheap, and it's certainly not going to be easy.

I concur with what others have said. There is no cost effective solution. If you will never rest easy without the aftermarket head unit, sell your GX and buy another one without NAV. A similar level-of-effort proposition comes up on a Dodge/Cummins truck forum I belong to; "Can I convert mine to a 4 x 4?," or "I want to change my single rear wheel to a dually." Of course any of these things can be done given time and money. But most people who have posed those questions just buy another truck after absorbing sage advice, and those that stubbornly engage in the uphill economic/labor battle rarely say it was worth it.

One thing to consider is that NAV equipped GXs are probably still more desirable outside of the enthusiast circles. Your average soccer mom would probably opt for NAV and pay a bit more for it. So if you were to sell and repurchase (assuming apples to apples condition/miles), it could probably be done in the vicinity of cost neutral, not counting any taxes, fees or b.s.

Well, here are the schematics for the HVAC with Nav:

And here are the ones without nav: