Gavroche is the son of the Thenardier family; Marius is a Pontmercy. They're both revolutionaries though Marius survives the battles and Gavroche is killed in the fight. Monsieur Thenarider and Colonel Pontmercy met by accident during a war.

Marius is brought up without knowing his father because he's been lied to by his grandfather. He doesn't find out the truth until after his father has died. Gavroche is ignored by the Thenardier family because they aren't interested in the boy. He actually gives himself his own name. He and Marius are compatriots who both believe in the same cause. Marius saves him once but ultimately Gavroche is killed.

Gavroche's father, Monsieur Thenardier, actually saved Marius's father, Colonel Pontmercy, after the Battle of Waterloo. The greedy innkeeper wasn't intending to save the Colonel and just happened to do so while stealing from the corpses left after the fight. Thenardier is not a good man but Marius gives him money to allow him to escape Paris because of the debt he and his father believed they owed him. While Thenardier pretends to be a Waterloo veteran, he and Pontmercy actually do not believe in the same cause.

The two pairs are similar in that they are both financial benefactors and supporters of the other individuals. Marius acts as a benefactor and takes care of Gavroche's needs. Pontmercy does the same for Thenardier—however, in a different way. In Marius and Gavroche's relationship, they are close and the arrangement is understood and agreed upon. Marius acts as a father figure as well as a benefactor, and Gavroche knows that Marius is helping him.

Thenardier, however, has no idea Pontmercy is his financier. Pontmercy acts as a silent benefactor, fueling funds to him so that he can continue his work and passion, while Pontmercy has to remain shrouded and unknown in spite of his love and care for the boy.