Well the Celts seldom attacked walled cities. But their lack of seige activity is likely more due to their social structure and its inability to sustain field armies except in times of extreme crisis.
At Cicero's camp, the Celts didn't use captured equipment, they constructed it themselves with aid from POWS and deserters.
At Alesia the Gauls 'prepared a great quantity of fascines, ladders and grappling hooks' (Civil War VII 81). Vercingetorix 'sallied with fascines, poles, sapper's huts, grappling hooks and other implements (ibid VII 84).
Now Caesar's defences at Alesia were impressive (as were Cicero's in his winter camp, the winter camps were considerably better fortifed than the marching camps).
Quite clearly the Gauls are capable of conducting offensive siege operations when necessary. The evidence is in Caesar. It is not lack of ability that prevented them from assaulting towns but lack of opportunity. Sensibly they attacked Caesar's armies in Gaul, not allied townships and they lacked the organisation to maintain a field army for long enough to invade Italy.
Also there is evidence on Trajan's column of the Dacians using siege equipment. If you really want me to I'll dig out some references.