I am new here. I think that since Gai won't be able to beat Madara, he would do some big damage to Madara before dying.
It could be, destroying Madara's Rinnegan.
Well He can deal enough damage to madara, even his SO6P black balls are sent to other dimension using kamui by kakashi so gai can deal easy damage to madara before dying but destroying his rinnegan is taking it too far....I personally dont think he'll be getting any idea as such let alone doing it
I hope he kicks some ass before he dies but realistically I think Madara will get serious and Gai wont even have time or strenght to do any more damage(could be wrong though)
Madar said hes excited he knows gai is on a level near his.hell do omething kishi wont let 8gates goto waste itll be be a nice legacy he leaves hes the only one currently he needs his credit