I think Garth being "the new Bobby" is just something he calls himself. Sure he knows a lot about hunting, and the guys do respect him for the most part- but Dean and Sam are closer to Kevin and they have to keep closer tabs on him than Garth.
Garth was trying to be "the new Bobby" sejak being the person most interested in Sam and Dean, the one they'd talk to about their problems. Not sejak being the sumber of all Supernatural knowledge.
