It's probably not just about the fuel economy, but the character of the engine. According to that Matty Maran video posted in another thread about the manual Gladiator, the 3.6L V6 doesn't have enough down low torque to hold 5th or 6th gear at 100km/h. You need to going quite a bit faster in order to be in the powerband of the engine before being able to sustain 5th or 6th gear.
The diesel likely fixes that.
Unfortunately, the manual isn't offered with it, so yeah. Moot point.