A filter in such a small space would be a very bad idea. These newts prefer masses of water that have the gentlest of currents or no current at all, and strong currents can cause stress and force the animals to abandon the water.
In a 10 gallon tank, or larger, a filter can be made redundant by using lots of live plants. You get the same benefits without the electric cost, the undesirable current and the heat production of the motor.
The recommendation for a minimum of 10 gallons is basically, because aside from the need for sufficient space, 10 gallons are aproximately the minimum volume required for adequate thermal stability (larger volumes fluctuate less, small volumes fluctuate very rapidly) and to prevent the rapid accumulation of toxic compounds like ammonia and nitrites, as well as buffering fluctuations of pH or hardness, etc...With less than 10 gallons, this parametres can vary rapidly, which is dangerous, plus maintaining adequate conditions requires a great deal of attention, whereas with a sufficiently large volume, maintenance can be greatly reduced allowing also for more stable conditions, which is greatly benefitial.
You can safely use fine sand instad of gravel, just make sure to have a thin layer, no deper than an inch to avoid the formation of anaerobic pockets. Alternatively you can have no substrate at all which facilitates feeding and cleaning.
The CC articles are linked in my signature, i strongly recommend reading them all with an emphasis on the water quality/cycling articles.
For further information you can also browse the help section for older, related threads, or the Cynops/Hypselotriton advanced section. You can also use the search function to locate threads that discuss specific topics.
Read, read until your eyes are sore!
