Absolutely, be cautious. If G is willing to be "the other" without your knowledge, then there's no respect for you there, and it's not likely that there will be.. that being said, your partner is showing you no particularly great amount of respect by allowing G to believe that she's cheating on you without your knowledge... personally, I'd insist on her coming clean with him asap, or breaking it off. What's she got to fear? If he's willing to be part of an illicit affair, why wouldn't he be part of something that didn't HAVE that stress attached to it (of course, I know that answer.. if it's cheating, that's understandable, but polyamory, what the fuck is THAT?).
If it were me, and I know it's not, I'd contact him myself (if she refuses to be honest) and say "look, I know you're dating her, but I actually am ok with that, as long as y'all keep it on the up and up with me, ok?".
I know.. freaky sounding, but I WOULD... I expose my polyamory pretty easily, so that's the way I'd go about it.. most likely won't work for many other folks, but I thought it was worth mentioning.
Then again, in our lives, if one of us meets someone, we come clean right away.. if they can't deal with our lifestyle, then they're not worth the effort of building a relationship with.