And the 33-year-old, a veteran of three Lions tours including last summer's trip Down Under, has been in fine form, playing a crucial role in the loose and winning penalties at the set piece.
And John believes it is not just his performances on the field that are having an impact at Cardiff Arms Park, but his attitude on the training paddock as well.
"Gethin has been phenomenal since coming back from international duty," John said. "He hasn't even had a break, we offered him a week off after the international series and he didn't want it.
"He has been captain, as Smiler [Matthew Rees] wasn't quite back, and he has been superb.
"He plays like a back row forward at the breakdown. He has played and trained with a smile on his face for the last four to five weeks and you can see that is paying dividends.
"What he does in training is demand high standards of other people. He is not afraid to tell people if they are doing the wrong thing or in the wrong place.
"Smiler, Bradley [Davies], Lou [Reed] when he is back playing and Gethin have been an example to them all and the boys are following them."