Even though they're small, gray tree frogs as I'm sure you know jump long distances and are very active at night. They will be very limited in a 10-gallon, sure you can keep one in there but for a little bit of extra money you can make the life of the animal so much better. Some books I've read have recommended a 10-gallon, whereas others it's been at least 20-gallons. I keep 2 green tree frogs in a 20-gallon and I wouldn't put them in anything smaller but it will come down to the keeper in the end.