This is a two-parter:
However, it also sounds like Brennus seiged Rome for about six months and was then paid 1,000lb of gold to leave - I didn't hear anything about them gaining access to the city, destroying records, etc. Though a fire was mentioned...
Am I correct in understanding it was this sack that led to the loss of records of history up to that point?
I'm probably misinterpreting the events, and hope to gain to some clarity on it. Thanks so much!
The only sack that makes sense is that of 390 BCE after the battle of the Allia. The Twelve Tables in particular were long gone by the 1st century BCE, and even copies had become hard to find. Cicero remembered memorizing the tables as a boy, but remarked that no one did that any more (de Legibus 2.59).
You are mistaken about the nature of the sack in 390. The Senones did in fact enter the city, as Livy tells it, through the wide open Colline gate. Most of the surviving soldiers had gone to Veii and the city was essentially defenseless. Livy's account describes how the Gauls butchered elderly patricians waiting in their atria and eventually made the bargain to get the holdouts out of the Capitoline fortress. Fires had been set in the city, but not as widespread as could have been.
I don't see anything in Livy's account about any list of kings or the twelve tables. It might be in the more dubious account by Diodorus Siculus or the even more dubious account by Plutarch. If so, I'd sternly warn about taking those claims at face value.