Gimli did not know that Balin’s colony in Moria had been destroyed because no word ever came back from it, not because he was ignorant of Khazad-dûm’s general history.
What Gimli Actually Knew
Gimli and the Dwarves of Erebor knew that Balin had gone to recolonize Moria and that, for a time, reports said the attempt was going well, with work restarted in the old halls. After those early, positive messages, communication simply stopped, leaving Moria’s exact status uncertain rather than confirmed as lost.
Why No News Reached Him
The most common in‑world explanation is simple: poor communication and distance. Messages between distant Dwarven realms were slow and dangerous, and any messengers trying to bring bad news from Moria could easily have been killed by Orcs or other foes before reaching Erebor. As a result, the silence created uneasy suspicion, but not confirmed knowledge of disaster, so Gimli could still hope to find Balin alive when the Fellowship passed through the mines.
Movie vs. Book Emphasis
In the films, Gimli’s ignorance is heightened for dramatic effect: his enthusiasm to see his cousin makes the discovery of the tomb more shocking and emotional. In the books, the Dwarves are already worried by the long silence, and Gimli does not push for Moria as eagerly; instead, it is treated more as a risky last resort, though he still hopes for good news.
