Hello!
So I can imagine what Romans might think of Gauls by the time they have conquered Cisalpine and Transalpine Gaul. But how about when Rome was but a city with several allied tribes? Were they merely known as "barbarians who come raid us every so often", or did they have knowledge passed on to them by northern Italian tribes (or even Etruscans for example who spend a considerable amount of time north side of Italy.)
The Gauls were definitely known to the Romans prior to their conquest of Cisalpine Gaul, as the Sack of Rome by Brennus left a deep cultural mark. Gauls/Celts were frequently employed as mercenaries by all major mediterranean civilisations (Carthage, Rome, the Greeks) and were known for their skilled metallurgy.
Roman trade nearly always preceded Roman military expansion; and though something resembling a true ethnographer didn't really exist as we know them from later imperial times (Tacitus, etc.) there would have definitely been stories and ideas about their culture.
The Germanic tribes were more obscure to the early Romans, as they did not live near the Alps for much of the classical period. Unlike the Gauls/Celts for example; the Romans never gave them an encompassing name. "Germanic tribes" is a modern invention; in Roman times it referred to a specific group living near the northwestern Rhine-border. The Goths for example, though they spoke a Germanic language, were not referred to as a "Germanic tribe" by the Romans at any time.