Sort of good news. Lexus (and Toyota) don't do specific builds. They standardize their production process by pre-determining what vehicle trims, options, and colors they will build, which reduces production variation to improve quality and/or cut costs. Dealerships get what they get regarding specific vehicles through an allocation process.
As a consumer, you either find the vehicle for sale on a lot that matches what you want and purchase it, or you contact a dealership to get added to their waitlist for when they get allocated a vehicle that matches your required specifications. For the GX specifically, it is more likely that you will need to join a dealer(s) waitlist, but depending on your geography or willingness to buy across state lines, you might be able to avoid that process.
The dealer waitlist process varies. Some require deposits; some will only accept people from specific geographies, some have multi-year waits, some charge above MSRP, etc.
The most straightforward next steps would be to call your local dealership(s) and browse the threads for how dealerships have operated in your area.
Hope that helps.