Gronk is easily the best TE in the game. Jimmy Graham is a glorified WR that does not play the position physically.
A look a Gronkowski and Graham at approximately the same time in their careers. Also a comparison of Gronk and arguably 2 of the best TE's in the NFL, Tony Gonzalez and Jason Witten.
Gronkowski: G/47, Rec. 211, Yds. 3,006, TD 40
Graham: G/46, Rec. 215, Yds. 2,648, TD 25
Witten: G/47, Rec. 188, Yds. 2,084, TD 13
Gonzalez: G/47, Rec. 168 Yds. 1,838, TD 15
As you can see, Gronkowski is way ahead of what some consider the best TE's of all-time, Gonzalez and Witten. Clearly ahead of even Graham, except in receptions and we know Gronkowski is way under used by the Patriots!
Also, Graham is already 27 years old after just 4 years in the league. Gronkowski is still just 24.