I love gray tree frogs, good find! 7 is definitely way too much for 20 gallons though. I have 4 of them in a 35 gallon tank. They aren't "social" frogs, so they want their space. Also, it would be best if you had something taller rather than wider, they don't like to be on the ground at all. None of mine ever touch the ground except for occasionally during hunting. Mine also like to bask, so I have a 50 watt blue daylight bulb on the top, and they find the branches close to it and perch themselves up there.

The sheet moss is okay, only if they cant get to the sphagnum moss. They are aggressive eaters and a mouthful of that could cause impaction. I do have a little in the background of my tank, but nothing they can get off.

Live plants is definitely an addition that should be made when you can. My frogs love to sit on anything with broad flat leaves and they just help with the general health of your vivarium. Make sure you clean the water daily, they like to use it as a bathroom.

Also, make sure you read the care article - Frog Forum - Gray Tree Frog Care

Most questions you have regarding habitat will be in there!