cwatters,
You're confusing the tug and the aircraft in this discussion....
In a tug, a small gas turbine might be a viable alternative in theory, especially since you're already on an airfield, so the fuel is available, and possibly maintenance expertise also.
In practice, diesel and gasoline tugs have been around for ages, so by the time you design the tug, and manufacture them in the relatively small quantities needed, all economic advantages are lost.
Also, if you think about it, trucks run long distances at steady speeds. If gas turbines were significantly better, they'd be more widespread.
The original point (this is for JediDude as well) was that jet engines (gas turbines) are hideously inefficient to drive a big vehicle (i.e., a jet aircraft) at very low speeds on the ground across an airport...... Not to mention that once you're standing still in a queue, your efficiency drops to 0% .... but you're still burning fuel, while a tug could turn off its engine for a few minutes.
The really infuriating thing about all this that in principle the idea makes some good sense......
Somebody mentioned burning 1.5 tons for a 60 minute sector (smaller aircraft, not a big airliner) and 100 to 200 kg taxying. On average, that's 10%!!
Engine and aircraft manufacturers spend billions to gain another 10% fuel efficiency.
When I first read the original news item, it took me only about five minutes to come up up with nearly all the objections listed here.
What DOES sound like part of an answer is far better flow control on the ground. It should not be beyond the wit of man to conceive a system whereby you start your engines, get pushed back, taxi to the runway entry, maybe wait one minute for the current incoming aircraft, line up and take off. Sitting in a queue with engines running means the flow management does not work.