Since the Shodai can only be wielded by a strong swordsman, I would have think that Gandhi would be at least low top tier.
I think he was above that in his prime , at least as strong as Mihawk
If we look at the other holders of 12 Supreme Grades so far , they are also some of the strongest characters :
β’ Mihawk himself ( Yoru )
β’ Whitebeard ( Bisento )
β’ Roger ( Ace )
But yes , now due to his age , i think he can be a low top tier
Reason why I don't say mid top tier or above is because if Zoro defeats an Admiral and move on to another top tier, Zoro won't have enough strength left to face Gandhi.
This Arc , Zoro went from fighting 2 Emperors and then went down , and then got up again due to that Mink antidote ( or whatever it's called ) and defeated King
So i think Final War Zoro beating an Admiral and then Shodai Gorosei ( who will possibly be stronger than the Admiral ) is something that's still possible
And the Admirals are stated to be the strongest fighting force of the World Government so I don't think Gorosei are above Admirals in strength.
Yes , the Admirals have been the Strongest Fighters of the World Government , above everyone else who were WG members , with only one exception that was Mihawk
Tho i wouldn't dismiss a possibility of the Gorosei being as strong as the Admirals ( or even above for Shodai Gorosei ) , by being turned to their prime ages by the time of Final War ,
Because someone who can alter people's ages ( Bonney ) have been tied to Mariejois