An average GAGer here is good talker. And even you know the reality because you written "And how do you know if a GAGer is a good listener?" on your question because you doubt their existence too (am I right?) Sadly most of the GAGers talk more than listening (literally, gives opinions more than receiving them) because they need just Xper points.
I can't vote both because I think both are true. People here I've found are both good at listening and giving advice, with the exception of the trolls. However some people dont get sarcasm. It feels so wasted. Makes me sad lol