The use of undergravel filters in tanks has been debated for years. Many say that they are not conducive to plant growth. Others say the added filtration is worth the effort. And still others, like me, see no difference when using them or not using them. I do, however, use them in a reverse filtration scheme and appreciate a significant difference over traditional flow. But a 20g tank is much too small for such a system.
Plants can be grown in just gravel, they may not, however, reach their full potential. There are amendments you can add to your substrate like flourite and laterite that have important micros, such as iron, and enhance vigorous plant growth. The texture of these products also allows rather free root expansion. Another product in the hobby is Eco-Complete for Planted Tanks. I use this as a substrate by itself and have had enormous success with it. I like the darker color also. It has been proven that darker substrate has a calming effect on you fish also.
When using gavel alone, you will ned to be on a fairly dedicated fertilizing regimen. In a tank as small as your 20g, I would recommend using liquid ferts, simply for their convenience. I have started using pfertz liquid fertilizers and am quite impressed with the results. They are also one of the forum sponsors. Alex and his crew have put together a four bottle package that includes both the macro and micro fertilizers that plants need. The product comes in user friendly squirt bottles and easy to follow directions. Some swear by Excel products, I have never noted great results with them, but they are effective. They are a bit more expensive. I also use plant tabs for the "nutrient hogs" in the tanks like that contain Amazon swords. These a large, heavy root feeding plant, and I think the tabs are a must for them. I use API tabs. They are economical, easy to use, and contain most essential elements. using the tabs and liquid ferts, in combination, all of the plants, whether root feeders or leaf/filter feeders are having their needs met.
Plants you could plant in your tank would include Java Fern
, some cryptocoryne species, Echinodorus sp. (sword plants), Anubius sp., elodea(a plant that can be anchored or floated) banana plants, and other medium to low light plants. Good luck.