Your best bet is another gray. In my opinion you shouldn't mix grays and greens unless you know what you're doing. Not a good idea for a new frog owner... Grays can out eat greens, causing greens to go hungry. I have my own setup with grays and greens in it. They have been living together for over a year. Nothing bad has happened, but I still wouldn't recommend a new frog owner to be mixing species, as they might not recognize signs of hunger or stress. Just my opinion though........