Julius Caesar, in his 'Conquest of Gaul', had a few things to say about the Germans.

• He found their chastity admirable, and praised them for their courage and honor.

• He thought they were lazy, inclined to laying about and brawling, and not much good for more than hunting, herding, and fighting.

• His personal theory was that because they were so big, they didn't get energy from food as efficiently. The lack of sunlight didn't help (something to do with agriculture and also with people being energized by it? I'm not so sure). Whereas Italians (that is, peoples of the Italian peninsula) were more compact, able to make better use of sunlight, and therefore more inclined to the organization and efficiency necessary for monumental construction, society, etc.

He also wrote about the Gauls, of course. I guess he found them brave but flighty and superstitious. It was a constant source of annoyance for him that they would get spooked while on the march by bad omens, changes in the wind, and other random signs. He seemed to regard the Belgae as brutal, fearsome marauders, but this was probably due to the Gauls' fear of them, and due to their ties across the Rhine with the fearsome, mysterious Germans. When he crushed them, he saw this as him doing the Gauls a pretty big favor.

The emphasis on superior technology is a modern Western preoccupation, a component of the 'Western Way of War' as put forward by V.D. Hanson (1989, 2001). It was not a particular preoccupation of the Romans, though their advantages from the Mid-to-Late Republic in siege warfare (learned from the Hellenistic Greeks) were considerable over tribal enemies such as the Gauls. However, even Caesar admitted the Gauls were quick studies in that regard. For more on this see Sidebottom 115-8.

From a military point of view (and the military aspect was the decisive component of Roman views of superiority over barbarians), the Romans according saw their superiority as revolving around disciplina, if not virtus. The virtus, or manly courage of general barbarian groups such as Gauls and Germans was hard to deny (and was routinely credited as impressive by Caesar), but they lacked Roman staying power and organisation in battle, and the Romans saw this as a result of their lack of disciplina. Disciplina was developed as a result of training and experience (Phang 38) but also had a moral component (resistance to luxury and laziness) and was viewed by the Romans as central to their military success over barbarian enemies (Mattern 203).

So the Romans essentially saw their superiority to the barbarians as cultural, military and moral, but not technological. Accusations of technical backwardness such as inexpertise in siege warfare was more of a reflection on their lack of cultural sophistication (lack of Hellenistic military science) rather than a purely 'technological' criticism. An obsession with technology as an explanation for military success is a modern pre-occupation, and not particularly relevant to the ancient world where technology was pretty similar across societies. After all, the Romans got their most common form of armour (the lorica hamata) from the Gauls: it was superior Roman organisation and resources that allowed them to better mobilise their technology on a large scale that made the difference, rather than the the technology itself.

However by the Late Republic, an elite (wealthy and/or highborn) Gallic warrior was equipped at least as well as a Roman legionary and, considering even the Romans grudgingly admitted they were extremely brave and capable fighters, one-on-one, the elite Gaul was without doubt at least a match for an experienced Roman soldier.

Sources:

Mattern, Susan P., Rome and the Enemy: Imperial Strategy in the Principate, University of California Press, Los Angeles, 1999.

Phang, Sara E. Roman Military Service: Ideologies of Discipline in the Late Republic and Early Principate, New York, Cambridge University Press, 2008.

Sidebottom, Harry. Ancient Warfare: A very short introduction, Oxford: Oxford University Press, 2004.

lots of answered questions similar to yours in the FAQ https://www.reddit.com/r/AskHistorians/comments/1xiz9h/did_the_romans_have_stereotypes_about_the/