I have new GM 2500HD on order and I went with the 18" rim/tire option in lieu of the OEM standard 17". I didn't put any do-diligence into considering the 20" rim/tire because I would be closing in on my garage door clearance height.
I'm somewhat old school in thinking, but I would think that GM has the 2500HD drivetrain (engine/transmission/axle ratio, etc.) optimized based on the "standard" components offered, i.e.; 17" rim/tires.
I remember back a few years Ford's publications stated that going from a 17" rim to a 20" rim reduced payload capacity by 500lbs....., I believe it was even reflected in their "build your own" tool. But since the start of the recent Big Three towing wars, I haven't run across this reduction statement.
I would assume that a 20" rim with a "larger tire diameter" (taller tire) then the OEM standard 17" tire diameter, the effective gear ratio would be reduced (IMO very little). I don't know the GM OEM 17" and OEM 20" tire diameter differences (if any).