Dam bro is really hurt this chapter.
Gaban did fine to.be honest.
As a person he is definitely great.
But the problem here is only of we compare his achievement to his fellow legend.
Whitebeard fought against whole marinford and defeated multiple opponent.
Garp fought whole pirate Island and needed shiryus fatal stab and Aokiji to go down.
Rayleigh completely stopped kizaru who let's not kind ourself is the fastest since he is light so not a slow person like Sommer. Then 2 years later stopped Blackbeard who is luffys nemesis.
People still downplay blackbeard but they will realize how ridiculously strong he is when he go all out and also the seriousness of the fact that he backed down when Rayleigh showed up.
Again it's not that gaban did bad its just that compared to his fellow legend what he did wasn't enough.
Oda likely purposely did this as to not have gaban outdo the other legend after all Whitebeard still look way better than any of them.