I thought I wanted one too. Pick-up without a roof? Yes please. But then I started investigating. The Gladiator and JKU are identical except for 20" of extra frame for the bed. The Gladiator frame is a Ram 1500 frame. (which means the JKU has the same frame but shorter) My JKU doesn't turn anywhere near as tight as my YJ. The Gladiator is even worse; turns like the truck it is.
But the worst part is the power train. (same motor, trans, transfer case, and axle ratio(s) 100% the same as a JKU. My max tow weight is 3,500lbs with my 3.73:1 axles. The Gladiators is 7,000lbs When my kids loaded up a U-Haul open trailer and put our three bikes in it, it was real close to that 3,500lbs. The Jeep pulled it . . . . barely. Just barely. I can't imagine doubling the weight. I don't think the Rubicon's 4.10:1 axles would make much of a difference.
The Gladiator was never offered with the Hemi, and I don't think they ever put their little diesel in either.