The Gauls were the inhabitants of Gallia / Gaul, a territory which roughly overlaps with modern-day France and Belgium (with a part of Switzerland too). They were mostly Celtic people. As a side note, it is important to note that there wasn't a unified polity of Gaul, and probably no sense of "national" identity -- certainly not in the modern sense. The people of Gaul were living within smaller social structures, and there were many peoples ("tribes" if you want, but be careful of the bias such a word carries with it when used in a profane way) like the Redonii, the Venetii, the Parisii, the Helvetii, ... the list goes on an on.

Our primary (written) source on pre-Roman conquest Gaul is, ironically, Julius Caesar himself, in several volumes where he (speaking in the third person) tells about his personal conquest of Gaul. The text is heavily biased, of course, serving personal glory and political purpose, but it has a lot of information for us.

The primary written source on Germanic people as seen by the Romans is a short book by Tacitus, Germania. He describes the social structures, rites and customs of the Germans. It is doubtful whether Tacitus himself went abroad; the book is likely to be a compilation of second-hand accounts. As with Caesar's Gallic Wars, there is an important amount of information within the book, but we have to be cautious with it. In the case of Germania, especially, since the book was used by German scholars in the 18th-19th centuries with the political and nationalist purpose of finding historical roots for the German identity. Such attempts were ultimately re-used within the national-socialist narrative in the 20th century. But I'm getting out of topic...

What do these sources tell us? Well, to start with, the Romans distinguished the Celts/Gauls and the Germans. The thought the Celts had fair (blond) hair, and the Germans had red hair. The Celts had agriculture and wrought iron, whereas the Germans did not. The Celts had "kings" (ri / rix, cognate with latin Rex of course), the Germans had several figures of power in their social hierarchy (kings but also chieftains).

The Romans distinguished the ritual pratices of Celts and Germans. The Celts had druids and the Germans had not. Tacitus says the Germans worshipped Mercury -- he assumes the German god Wotan (there are various spellings) was an avatar of the Roman god Mercury (interesting etymology side note: wednesday comes from an old form of "wotan/woden's day"; French mercredi comes from an older form for "the day of Mercure").

I'll come back to this comment for more sources as soon as I have my books at hand.

I thought the Gaul were Celts. If they were what was the distinction between the two? The area where they lived?

I just assumed this is where Wales/Gales and Gaelic comes from. Can someone clear that up if it is in the scope of an answer to the question of OP?