The Holland international has been impressive for the Reds since his August arrival.
Hamann, who played for both clubs, told Tz: "Gravenberch is doing great at Liverpool so far.
"It is therefore almost unbelievable that he hardly got around to playing at Bayern under two coaches (Julian Nagelsmann and Thomas Tuchel).
"But now at Liverpool you really see what Gravenberch is capable of. He is physically strong, can score a goal and has a certain speed. When you see him so busy at Liverpool, you really wonder why he wasn't a regular at Bayern."