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.
It told how Balin discovered Durin's Axe, and established a small colony, but it was overrun by orcs and Balin was killed by an orc archer in Dimrill Dale. Thus he died in the same place as his father, having been self-proclaimed Lord of Moria for less than five years.
Why Didn't Gimli Know About What Happened In Moria? (Khazad-dum) | Lord of the Rings Lore
Is Gimli a descendant of Durin?
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.
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.
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.
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.
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.
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.
By the end of the Third Age, Moria had long been abandoned by the Dwarves, and was a place of evil repute. It was dark, in dangerous disrepair, and in its labyrinths lurked Orcs and the Balrog.
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. '
The name “Durin's Bane” literally means “that which destroyed Durin”, so it's fairly certain that the name referred only to the balrog in Moria. The dwarves didn't know what it was, since none of them had ever actually seen it. They just knew something they had never encountered before had killed Durin.
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.
Gandalf's main reason for supporting Thorin's quest in The Hobbit was to eliminate the threat of the dragon Smaug, who would be a powerful ally for Sauron. Despite popular belief, the Balrog known as Durin's Bane was more powerful than Smaug in Middle-earth, being a literal, primordial demon.
It is also stated in Unfinished Tales that Gimli was prevented from traveling with his father on the Quest of Erebor because Thorin and Company thought him too young, though Gimli, being in his sixties, considered himself ripe for adventure and was disappointed to be left behind.
As in the latter case, the Elves were too busy defending their own homes from Sauron's (and Saruman's) growing power and couldn't spare their dwindling fighters to help the Men. Elrond's sons, Elladan and Elrohir, also defended Gondor during the Battle of Pelennor Fields.
Gimli is not the last of the dwarves. This is a common misconception because Gimli is the only dwarf who gets a fair amount of coverage in the franchise. Lord of the Rings focuses its narrative primarily on the affairs of men, elves, and hobbits. Dwarves are a secluded race that lives in the mountains.
