Roman writers wrote often about the Celts, and in their depictions of the Gauls, it does fit a more 'Nordic/Aryan looking' people.

Didorus Siculus says:

The Gauls are very tall with white skin and blond hair, not only blond by nature but more so by the artificial means they use to lighten their hair. For they continually wash their hair in a lime solution, combing it back from the forehead to the back of the neck. This process makes them resemble Satyrs and Pans since this treatment makes the hair thick like a horse's mane.

(This translation can be found on page 12 of The Celtic Heroic Age, ed. John T. Koch and John Carey. Aberystwyth: Celtic Studies Publications, 2003.)

It must be kept in mind, however, that the accuracy of these accounts are unknown. Posidonius certainly was in contact with the Gauls himself, but writers such as Diodorus Siculus may have culled their writings entirely from his accounts, without ever having visited Gaul themselves. Caesar certainly was in Gaul, but as his accounts are very similar, it has been suggested that he too, relied heavily on Posidonius for De Bello Gallico.

For the most part, these writings have to be taken with a grain of salt, so to speak, as the motivations of these authors was to depict the Gauls, and the Celts as a whole, as a race of barbarians in desperate need of conquering. It is believed that their depictions are exaggerated.

I am no expert in Roman custom of belief, but I personally cannot see any reason why the physical descriptions of the Gaul would be exaggerated to demonstrate their barbarity. Their violence? Yes. Their system of religion and governance? Yes. But their physical appearance? I'm not so sure. So, I think there's a decent chance they were blond and tall, but there's no way (that I know of) to know for sure.

For some further reading:

Cunliffe, Barry. The Ancient Celts. London: Penguin Books, 1999.

Tierney, J. J. "The Celtic Ethnography of Posidonius." The Proceedings of the Royal Irish Academy 60 (1960): 189-275.

Williams, Derek. Romans and Barbarians. London: Constable and Company Ltd, 1998.