lol I was just about to post, saying that Rabbut might be right as upon closer inspection of the original picture it does look different at the base of that valve.... and you've kindly posted a close up
Its not in fact a JG fitting then but a flush valve, so you are fine.
That chrome fitting is a washing machine tap valve. All you need is the 1/4" adapter and you can hook it up.
If you have room under the sink, you can mount the RO unit underneath and tap into the waste pipe for the waste water. Alternatively, you could drill an 8mm hole through the wall and run the 6mm pipe through it to the outside and use it for watering your garden or something.