I wouldn't put anything else with such a large gecko in a tank only 20 gallons. As for plants, I think you could do pretty much any of the standard terrarium plants. F. pumila, bromeliads (might want to avoid ones that have spikes/spines on the leaves), moss, fittonia, a sturdy fern (like lemon button fern), selaginella, etc. I'd use bark for the substrate. The plants can be planted directly into the bark, they like it. No need to hide the pots. Is the tank just bare right now?

I agree with sahoy -- don't add any other reptiles or amphs w/ a tank that size. When choosing plants, try to pick ones that will enjoy the same type of environment as the gecko. I am unfamiliar with the needs of the golden geckos or I'd try to be more specific. Sorry about that.