Hi Susan,
Undertank heaters will not radiate heat downwards the way a Helix, ProProducts, or Desert Rays heat panel will.
I've heard of of UTH's and Flexwatt being placed on the back (not on the ceiling) of enclosures to create warm spots for arboreals but this is not exactly ideal, especially if these products will be depended on to provide the majority of heat to the cage.
So if your cages will be in a cool room, or a room that will be cool for part of the year, I think that using UTH's or Flexwatt would not be a good idea.
A better use for these products in a GTP enclosure is to place them under the floor of the cage and set a large water bowl over that area of the floor. This will cause the water to evaporate quicker, therby raising the air temperatures (moist air is easier to heat that dry air). I know of at least one well known GTP breeder who utilizes this method. However, he uses long cages and places the heatpand/bowl at one end of the cage to creat something of a gradiant.
Keep in mind it's sucess will depend on the temperature of your room, cage material, and amount of placement of ventilation. It will not provide much of a thermal gradiant compared to a radiant heat panel, especially in cage that is not very long (such as your 2x2's).
So with all that said and not knowing your exact conditions, I'd guess that a light bulb or heat panel is a better choice. But there is a chance you could get an UTH to work for you. Just don't mount it on the ceiling of your cage.
Also, the Desert Rays heat panels have not received very good reviews. When it comes to heat panels, Helix seems to be the way to go.