Germans, Romans and Celts

The technical concept "Germans"
The English called the area North of the Alps "Germany". That is confusing, for the English themselves, "Anglo-Saxons" are also descendants of German tribes, as are the Dutch and Scandinavians.

Celts are the first people entering France that acquired a more or less dense and continuous spreading of population of tribes maintaining diplomatic ties. Celts were the people Caesar found when he conquered France ( Caesar shaping Europe Caesar about Celts (Gauls)).In the centuries before they had spread themselves over what now is called Turkey, Bulgaria, Yugoslavia, Austria, Switzerland, South Germany, Northern Italy, France (except the far south and far north), England and Ireland. Pretty tough warriors in early stages, but no builders of an organised imperium. Those of Northern Italy pillaged Rome once and burned it down in 390 BC. But they always just went home afterwards. They did form an organised religious system, with general druid conferences also settling conflicts among Celtic nobility on land, property and marriage issues.

The "French"
The present population of France consists of Celts mixed with people who entered France in the times of Roman domination, and later, a small group of Franks (Germans) who took power in the early middle ages. The name "France" suggests a lot of Frankish  (that is: Germanic) blood, but wrongly so. After the fall of the Roman empire a relatively small tribe of Franks overpowered the Romano-Celts in France but had to be taught how to rule it (Gaul had adopted a lot the complicated governmental and juridical procedures of the Roman empire). Hence, the Franks were unable to impose both their rule and their language (old-German, as still spoken by Frankish tribes in Hessen, think also of Frankfurt) and so adopted the language of the conquered.
Hence: the treasured French "francofonie" does not originate from Frankish language, but from Latin as spoken by Celts. In Roman times France was an exercise in administration for youngsters of Roman patrician families, and Celts were trained in Roman administration, law and making the Roman kids believe they ruled Gaul. This gave Celtic elite excellent routines to deal with the hordes if primitive Franks when these arrived. Around 500 Clovis could immediately sit down on one of the chairs left by the Roman elite trainees and be taught in much the same way by the learned Celtic elite how to rule Gaul.
This exact genetic mix of the French is unknown. To me, something like 60% Celtic, 20 % German (only part of which Franks), and the rest "Roman" (but see below, "Roman" is a hot mix itself) and Aquitani, whose home ground, from centuries before Caesar, was the south west corner of France.
Complete Celtic and Germanic tribes got massacred in the wars in which the Celts conquered the area, and later when Caesar conquered Gaul, chasing Germanic tribes and non complying Celts, and again later, after Germanic tribes had penetrated. 
What did it mean if your tribe seriously lost a battle? The men defending your tribe would be partly killed, partly taken and sold as slaves, and partly may have succeeded to disappear without a trace, they were not likely see others of their tribe again. Women, as far as thought attractive, would be taken also, children and older people left to die or join a related tribe in the vicinity.
Armies of Celts, Germans and Romans were usually accompanied by traders buying the slaves and spoils, as far as not taken home for triumph. The German tribes were the only ones who had no slave traders of their own, since trade was thought to be despicable in German culture. They sold their captives to foreign merchants.
Obviously, the wars with its production of large amounts of fugitives and slaves did considerably mix the peoples searching for home ground in France.

The Romans started as a small tribe ( "Hostage Syndrome": Livius on the Sabine Virgins), and had to dilute the concept "Roman" to get enough people for their armies. For soldiers to be effective, it was thought, they needed  more than pay and beatings: civil rights, that is formal Roman citizenship. Thus, early leaders liberally sprinkled Roman citizenship titles over the crowd of Celtic and even Germanic immigrants and asylum seekers.
In the third and second centuries BC, the red haired Celts had such a reputation in warfare that the Romans decided to copy them by wearing a kind of red haired brooms downside up on top of their helmets.
High patrician families kept themselves closed to outsiders. The almost exclusively intermarried. But the licentious sexual inclinations of the patrician women allowed the strong young German slaves in the patrician households to considerably refresh the genetic pool of the Roman elite. The Romans turned bastarddom into the foundation of elite culture. In the end, from a DNA point of view, the "Romans" became a healthy mix of all Mediterranean and northern European tribes.
Tacitus himself, the main source of the conclusions above, originated from a family in the Province and thus probably was mainly Celtic or Aquitanian, though there was also al lot of blood there coming from the Greek colony of Marseilles.

More about Gauls and Germans in Caesar shaping Europe