Here's another instance of what appears to be Gallic sacking/besieging - from Polybius book 3.
"Scarce had both these colonies been established when the Boii Gauls, who had been for long as it were lying in wait to throw off their allegiance to Rome, but had hitherto found no opportunity, elated now by the messages they received assuring them of the near arrival of the Carthaginians, revolted from Rome, abandoning the hostages they gave at end of the former war which I described in my last Book. Calling on the Insubres to join them, whose support they easily gained owing to their long-standing rancour against Rome, they overran the lands which the Romans had allotted to their colonies and on the settlers taking to flight, pursued them to Mutina, a Roman colony, and there besieged them. Among those shut up there were three men of high rank who had been sent to carry out the partitionment of the country, Gaius Lutatius, a former Consul, and two former Praetors. On these three requesting a parley with the Boii, the latter consented, but when they came out for the purpose they treacherously made them prisoners, hoping by means of them to get back their own hostages. When the Praetor Lucius Manlius, who with his troops was occupying an advanced position in the neighbourhood, heard of this, he hastened up to give help. The Boii had heard of his approach, and posting ambuscades in a certain forest attacked him from all sides at once as soon as he reached the wooded country, and killed many of the Romans. The remainder at first took to flight, but on getting to higher ground rallied just enough to give their retreat an appearance of order. The Boii following at their heels shut this force too up in the place called Vicus Tannetis."
I haven't found anything actually describing gallic seige equipment or tactics though.