I've never towed in the Rockies so have no experience to help with that decision. From what I've read, gas can do it, it's just slower, louder, and descents are more "fun"
A lot depends on how much time you plan to spend in the Rockies, and where.
If you're going to be in Colorado for an extended period of time, a gas engine will not be very happy at higher elevations and you can lose a lot of power. We have many passes out here that are in the 10000-12000 ft range. Other states in the Rockies, not so high, but still can be steep inclined/declines.
Grades will be another thing that a gas engine will not be happy with. There are plenty of grades in Colorado that are in the 6-12% and more range. Pulling upward at length on a 12% grade with a gas engine, you'll be "maybe" 20-25mph and you'll just watch your fuel gauge plunge. Traffic will line up behind you and you'll be looking for the slow vehicle pulloffs frequently. Going downhill is not fun...not fun at all...with a gas engine.
In the flats/lowlander/plains areas, you'll still get better fuel economy with a diesel. Having driven across the Great Plains and down to the Deep South with a gas engine and 6000lb trailer, I only managed maybe 9mpg, and had to stop every 200miles or so for fuel. With the diesel I now have, on a trip through the desert of northern Arizona, I was still able to get ~12mpg. Leaving for another trip across the Great Plains soon, and I'm anticipating getting 12+mpg with the diesel.
Personally, I would never go back to a gas engine for any sort of towing that might remotely be in the mountains or a sparsely populated area. And keep in mind that, with a gas engine and your projected trip to Alaska, fuel is going to be harder to find along the way, unless you've got an aux tank.