Combining the poor communication between Erebor and Moria with the Dwarves' extended lifespans, Gimli might not have known about the fall of Moria simply because Erebor had not grown too worried about the lack of information yet.
Gimli didn't know about Moria's fate because there had been no communication between Erebor and Moria for 25 years, and the Dwarves were hopeful and did not want to accept failure until there was certain news.
Why doesn't Gimli know the password to the Mines of Moria? Gimli didn't know the password to the door because that knowledge was lost. It was too long ago when dwarves had lived there. Elves built the door, back when they were friends with the Dwarves, thousands of years beforehand.
Gandalf didn't know that the Mines of Moria had fallen. He mentions he had been through Moria before, but it was before it had been resettled, which means it would not have had the chance to be overrun yet. All that was known about the mines was that the dwarves had gone silent.
Yet, somehow, Gimli was unaware of Moria's fall until he discovered Balin's tomb. All he knew was that Balin's mission was initially successful, which is why he suggested the Fellowship should travel through the Mines of Moria: He wanted to visit family.
Why Didn't Gimli Know About What Happened In Moria? (Khazad-dum) | Lord of the Rings Lore
Is Gimli the last Dwarf?
Though Gimli has no children, he is not the last Dwarf of Middle-earth. After the fall of Sauron, his people continued to thrive in the Lonely Mountain. Gimli made his home in Rohan, in the Glittering Caves of Aglarond, and many of his kinsfolk came with him.
From TA 2989 to TA 2994, the re-colonization of Moria was an attempted expedition led by Balin to re-take the ancient Dwarven Kingdom of Khazad-dûm. An initial effort was successful in reclaiming some of Moria from the Orcs.
The Balrog killed Durin VI, the Dwarf-King of Khazad-dûm, whereafter it was called Durin's Bane by the Dwarves. Avarice, principally for mithril, drove the dwarves to go too deep and awaken the Balrog.
When The Fellowship is taken to Galadriel, she and Boromir have a telepathic conversation which immediately causes Boromir to burst into tears. She senses his desires for the Ring but tells him not to give in because even now, there is hope.
According to one theory (via Stack Exchange), Gimli did not know about the fall of Moria in The Lord of the Rings: The Fellowship of the Ring movie due, effectively, to poor communication.
Why didn't he? He had been through Moria (more than once), but if memory serves he had only gone East to West, never West to East. So when he came to this place before, he may not have realized it was a fork in the road as he was coming out if the fork, not into the fork.
Therefore, the only explanation as to why Gandalf doesn't reveal the Balrog's presence to the others earlier in the films, is that it is a plot device used by the writers and the creative team in order to increase the dramatic tension of Frodo's decision.
In the end Tolkien stated that there were probably "at most" seven Balrogs: In the margin my father wrote: 'There should not be supposed more than say 3 or at most 7 ever existed. '
Gimli was born in the Ered Luin in the Third Age, son of Glóin. Gimli had wanted to accompany his father on the quest to reclaim Erebor, but at age 62 he was deemed too young. He was a remote descendant of Durin the Deathless, chief of the Seven Fathers of the Dwarves and ancestor of the Longbeards.
Balin was distantly related to him, and a descendant of the noble House of Durin. He later went to re-establish the Dwarf presence in Moria, but within five years it was overcome once more by Orcs, and he was killed there along with the rest of his companions.
Smaug could easily have cut through the armies of Mordor, and delayed the invasion of Middle-earth by years. So it wasn't in Sauron's interest to take on the dragon, but that doesn't mean he saw it as a lost cause, as he fully planned on creating an alliance.
So, when Gandalf agreed with Frodo's decision to travel through Moria, he knew that something could be hiding there. Yet, he had no reason to think that it would be something as awful and powerful as a Balrog. Only King Dain Ironfoot knew that Durin's Bane was a Balrog.
The company fled with Gandalf, but the Orcs and the Balrog, taking a different route, caught up with them at the Bridge of Khazad-dûm. At the Bridge, Legolas instantly recognized it as a Balrog, and Gimli recognized it as Durin's Bane.
In the fictional world of J. R. R. Tolkien, Moria, also named Khazad-dûm, is an ancient subterranean complex in Middle-earth, comprising a vast labyrinthine network of tunnels, chambers, mines and halls under the Misty Mountains, with doors on both the western and the eastern sides of the mountain range.
He proclaimed himself Lord of Moria. For five years the colony thrived. They managed to find many old treasures such as Durin's Axe, helm, mithril, and armouries. But on 10 November T.A. 2994, as Balin went to look in Mirrormere, an orc archer fatally shot him.
