Correction
When Aokiji joined the navy he was called the recruited monster and possessed a great fighting force, so at first he didn’t have anything to learn from the marine, but he later gained more power through Garp’s teaching.
Aokiji was looking at Monkey D. Garp's back(not past) since Aokiji was a new recruit.
The meaning of that part is
Kuzan respects Garp’s free way of acting and that he acts according to what he believes in.
After the Rocks Pirates disintegrated, Whitebeard gathered a crew and expand his territories/influence,
“After the destruction of Rocks Pirates, who was once called the strongest on the sea, its remnants gathered comrades and expanded their territories/influence. Among them, Whitebeard who is strong and charismatic is the one that stands out.”