Right around the time of the Roman conquest of Gaul, contemporaneous accounts describe the Gauls as having looked like beasts or savages, with thick knotted cloaks and unkempt long hair and beards. Roman depictions of the day clearly came with a victor's bias, with the Romans having both social and political reasons to depict the Gauls as brutish or uncivilized, further legitimizing Gaius Julius Caesar's claims of "civilizing" the Gauls.
Ammianus Marcellinus wrote in his Roman History, available from Gutenberg:
Nearly all the Gauls are of a lofty stature, fair, and of ruddy complexion; terrible from the sternness of their eyes, very quarrelsome, and of great pride and insolence. A whole troop of foreigners would not be able to withstand a single Gaul if he called his wife to his assistance, who is usually very strong, and with blue eyes; especially when, swelling her neck, gnashing her teeth, and brandishing her sallow arms of enormous size, she begins to strike blows mingled with kicks, as if they were so many missiles sent from the string of a catapult.
Dionysius of Halicarnassus, in Roman Antiquities XIV, available from UChicago, said both:
The Gaul was a tremendous creature in bulk, far exceeding the common build.
Now the barbarians' manner of fighting, being in large measure that of wild beasts and frenzied, was an erratic procedure, quite lacking in military science. Thus, at one moment they would raise their swords aloft and smite after the manner of wild boars, throwing the whole weight of their bodies into the blow like hewers of wood or men digging with mattocks, and again they would deliver crosswise blows aimed at no target, as if they intended to cut to pieces the entire bodies of their adversaries, protective armor and all; then they would turn the edges of their swords away from the foe. On the other hand, the Romans' defense and counter-maneuvering against the barbarians was steadfast and afforded great safety.
Clearly unbiased sources. I'd imagine paintings of the day aren't a particularly good source either, so I'm really looking for info from a source which had little reason to be biased, or secondary sources that take efforts to find accuracy in the inherent noise and bias involved. What did the Gauls look like, as far as hair, clothing, facial hair, and the like, around the first century BC?