Gu Fei was well aware of Gu Xian's capabilities. While the man did not train his kung fu, that definitely did not mean that he did not know anything about it. It was merely his physical limitations, technique execution, and power that made him unable to compete against Gu Fei. However, now that they were in a game setting, their physical attributes were transmuted into stats. As such, if they were to fight, Gu Fei would instead be the one at a disadvantage due to his Mage physique.
Gu Fei knew this, and Gu Xian looked to be well aware of this fact as well. Otherwise, given the body Gu Xian had in real life, a slap from Gu Fei would leave him flying for three days, so saying something so face-slapping might well have resulted in him being slapped to death. Why else would the man trouble Gu Fei less and less in recent years? Still, this was not a certainty here in Parallel World.