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A lot of manufacturers are going with 6-speed boxes with overdrive ratios that are similar to older 5-speeds. They don't need longer 6th gears because fuel economy is tested within very limited speed windows, so why bother?
I'd look and see if the Genesis sedan has longer gearing and whether surplus differentials or gears are available, because no way is a new custom gear going to be cheap... and I doubt you'll find a longer final drive from performance parts makers. To get longer gearing, you may have to go for taller tires.
The longer gears that I know that are available on the market for common cars are simply repackaged gears from Euro- or Japanese-spec cars with different engine choices from US cars. Far as I can recall, the Genesis gearbox is the same wherever you are.
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