I am not sure why J said not to say anything, but I guess it depends what J was thinking about when J said not to say anything and why.
I also seems to me that this is purely you taking an interest in some friends, so this is no skin off your nose (directly) if you do nothing.
That said, saying something or doing nothing are both valid options, although saying something can backfire or it can do wonders. Doing nothing is just... stepping back and observing while "nature" takes its course.
(Mind you, if you feel like C and G would make a nice couple, that is your opinion. It also may be, unfortunately, only your opinion.)
If you feel so inclined to say something, let me point out just three possible options:
1) Walk up to C. "You, C! G digs you! I know, she told me!"
Direct, but J DID ask you not to say anything, and this definitely qualifies. J might well be upset with you, especially if C screws it up and offends G in the process. Also, if C and/or G got word that one of them told you that they liked the other, and you used that to place matchmaker... that might get them upset with you, because they happened to have their eye on someone else.
On the other hand, like I said, direct. Simple. In a way, elegant, like a simple chemical equation for an explosion.
2) Wait until C mentions something about G or C's romance life. Say, "Hey, um, C? You didn't hear this from me, but I think G likes you. Just sayin'."
A little more subtle, but it offers a wet paper towel for a defense against anything J might say about you not saying anything. Again, J might get upset with you. More or less, just see the resolution above, only with about half the volume of volative mixtures.
3) Talk about something which both C and G like, a common hobby, hopefully one which C knows G likes, and one that you like as well (you are all friends, so it's possible). Mention that you will be leaving soon and that C will be on his own to find someone to hang out with that knows this hobby. Say no more.
Subtle, which means it might not work. But then, that might be all the nudge C needs. If it works, you can point out to J that you didn't say anything at all to C about G (True).