A filter is only 20-30 dollars american. A submergible or ungergravel filter is not 100% neccesary, hover here are some things to consider.
1. You wont have to clean the tank as often. If you buy somthing with a foam insert, you clean it mabye once every 10-12 days.
2. Newts don't like change, and cleaning gravel and removing them can cause stress.
3. Consider that the plants may not do so well, espically if you are cleaning once a week. If you have more then 3 newts and you are feeding them properly the water is not going to work unless you filter it, beacuse changing things around kicks up gross stuff, that will settle later, but needs to be removed.
5.Buy a submersable filter. I live 10 minutes from the border and will gladly bring you one.