Personally, I would return the ammonia gauge as they tend to be inaccurate to the point of uselessness. Nothing trumps a good liquid test kit like this one: http://www.fosterandsmithaquatics.co...54&pcatid=4454
although since you're not filtering and cycling the tank, you will only be needing the ammonia tester until the point that you upgrade.
What kind of heater did you end up getting?
Please keep in mind that you should be doing 100% water changes and cleanings every 4 days in a tank that size. Because the only safe quantity of ammonia is 0 ppm, the only way to maintain this reading is by doing frequent 100% water changes in uncycled tanks.