If we look back at Caribou’s adventure in the New World we are introduced to a Revolutionist Commander by the name of Gaburu.

Caribou’s adventure took place on an Unknown Island that was controlled by Kaido. The same island that X-Drake attacked and as a result, somehow is working for Kaido now.

X-Drake coming to the Island

Gaburu’s revolutionary soldiers are on this island and there are workmen there that seem to be forced to work in factories to create weapons for Kaido.

I believe that Gaburu went against Kaido somehow.
But why would a revolutionist commander go up against a Yonko?!

I believe that Gaburu’s Revolutionist Army was affiliated with Dragon’s Revolutionary Army and both worked to take down the WG. But both probably differed in ideologies.
Seeing people be enslaved by Kaido and forced to work for him, Gaburu sought to free these people.

I think that when Gaburu was defeated by Kaido’s crew but I believe that he didn’t really die and somehow went missing

Reasons I think this:

1) Isn’t it weird that Gaburu’s men were on this island looking for him?

This may mean that they are not sure whether he is dead or alive.

I also think that they don’t know that Grammy visits Gaburu’s grave.
It was only shown that Caribou’s men seeing Grammy at the grave.
Although, they will probably know now.

2) One Piece Manga chapter 731 is the same chapter where we find out that Sabo is actually alive and in Dressrosa and not dead. So could this relate to Gaburu’s supposed death and how he is actually alive?!

I believe Grammy may have witnessed Gaburu’s defeat by Kaido’s men but did not see him get killed and thought he died or she for some reason found some reason to think he was dead.

This leads me to believe that Gaburu may be one of the Revolutionary army leaders that Dragon wants Koala to assemble

Since we already know from previous theories that Gaburu’s appearance is based off of Che Guevara, an Argentine Marxist Revolutionary and a major figure in the Cuban Revolution.

Che Guevara would travel around the world and influence others through his Marxism philosophies and gain supporters.
Che Guevara would also ally with other revolutionaries and radicals such as Fidel Castro.
This could parallel to how Gaburu may have some connection with Dragon.
https://en.wikipedia.org/wiki/Che_Guevara

Conclusion:
Gaburu went missing after a confrontation with Kaido’s men
Gaburu became one of the leaders of the Revolutionary Army

Hope you liked my theory!

*Theory by Primordial Sky