This whole story started as people playing a game and then getting pulled into the game unable to leave. It seems like this may be a real thing happening to the players. The game seems to be something that is somewhat controlled by the guides who were once G**'s. Their aim is to destroy the big monster thing which wants to eat planets. 

 

It seems like the guides designed the game so floors 1-89 are preparation to give each player and their kingdom a fighting chance against the monster. 

 

This process has occured 41 times before and the current story is the 42nd generation. Each generation has had a number of kings and kingdoms which increased with each generation. It seems like each planet was home to a different race of creatures which were then added into rotation with the defeat of every generation, meaning that the number of players/kings increased with each generation. Also each generation is a planet that was being threatened by the big monster thing and was ultimately consumed by it because the previous kings were unable to defeat it.

 

Finally it seems like the grand dukes are people who were once in the king/player position but were unable to save their planet. Once their planet is consumed the kings with the most resentment and determination are able to take the position of grand Duke. 

 

There's definitely more that was reveleaed but that's what I understood the best.