What program would be appropriate for designing a reimplementation of the TurboGrafX16 at gate level?
At Nvidia, they probably use cadence. I think that's pretty much the only option when it comes to designing digital circuitry of that scale on cutting edge processes. But don't bother trying to use that unless you are taking graduate level digital design courses and have access to the software through a university or similar, the software is incredibly complex and licenses run into the hundreds of thousands of dollars per seat. And for that kind of design, you're usually working at an RTL level or higher, so no schematics.