Gimli and the other Dwarves lived in the kingdom of Erebor and, since Erebor was far away from Moria, news would have been slow between the two kingdoms. The journey to Moria was dangerous, so many messengers could have died going to or from there.
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.
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.
He was a descendant of Durin the Deathless, father of the first house of the Dwarven people. Gimli was descended from, but was not a member of the royal line. Through his father, Gimli was also the first cousin once removed of Balin, Lord of Moria, and his brother Dwalin, two more former companions of Bilbo.
Why Didn't Gimli Know About What Happened In Moria? (Khazad-dum) | Lord of the Rings Lore
Was 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.
Simply put, he was the only one available. There were only two dwarves at the council of Elrond, Gimli and his father Gloin. The Dwarves were fighting Sauron's forces constantly in their own campaign and couldn't spare many. Gloin was also much older now, it was absolutely his son's turn to go on a big journey.
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. The Dwarves attempted to fight the Balrog, but its power was far too great for them.
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.
However, a specific scene in The Lord of the Rings has led fans to theorize that Frodo and Legolas didn't talk, not because of time, but because the hobbit didn't know Legolas' name (and was too embarrassed to ask).
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 TA 2941, Gimli was prevented from traveling with his father on the Quest of Erebor because Thorin and Company thought he was too young, though Gimli, being in his sixties, considered himself ripe for adventure and was disappointed to be left behind.
The Hobbit dwarves Óin, Balin and Ori all die prior to The Lord of the Rings, when a colony is sent from Erebor to reclaim the Mines of Moria. Óin is killed by the Watcher in the Water, Balin is shot by an orc, and Ori dies in Balin's Tomb, his corpse featuring briefly in The Fellowship of the Ring.
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.
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.
The Balrog, like Gandalf, is a Maia. There is a very real possibility that Gandalf will die in this confrontation. As an istar Gandalf is an incarnated maia, and thus subject to all the frailties and fears that Elves and Men are subject to.
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 only recorded instances of Balrogs having been slain are Gandalf's slay of Durin's Bane, Echtelion's slay of Gothmog and Glorfindel's slay of an unnamed Balrog.
For more than five millennia, the Balrog hibernated in his deep hiding place at the roots of the mountains in Khazad-dûm. He remained undisturbed throughout the Second Age and most of the Third, before the mithril-miners of dwarf-King Durin VI awoke him in T.A. 1980.
Unfortunately, the Fellowship's journey only confirmed what he had feared. Although Gimli knew that Moria could be dangerous in the book, he did not know about Balin's death in The Lord of the Rings: The Fellowship of the Ring movie possibly due to slow communication and Dwarves' perception of time.
Gimli calls Legolas “lad” because the elf is more youthful than him in appearance, if not in actual age. (Note: it is only an educated guess from readers that Legolas is at least 500 years old. Tolkien never specifies his age.)
