First of all, "Gaul" is the name of ancient France. When Rome ruled Europe, the Romans called the natives living in France Gauls. Later, after the baptism of history, although Roman culture was replaced by Christian culture in the Middle Ages, the name "Gaul", which originated in Rome, was adopted by the people in the region (that is, the French at that time) and has continued to this day, which can be regarded as a part of French historical and cultural heritage.
Secondly, why should the animal "chicken" be added after "Gaul"? Originated from the French worship of a local "chicken". It is said that "Gaul" is pronounced as "rooster" in Rome, while "Brest Chicken" which happens to grow in France is very brave and aggressive. Over time, the French also regard "Brest cock" as a national symbol. In the French Revolution, "gallic chicken" was even printed on currency, and later it was more and more used in other items representing France.
In addition, France also has a nickname called "Marianne", which is said to describe France as a beautiful woman with artistic knowledge. There is also a saying that her image comes from the Marseillaise sung by the sculptor Francois Rude and the Angry Rhea on the Relief of the Arc de Triomphe created during the July Dynasty. However, this nickname is basically unknown to domestic people, and the sense of existence is not as strong as that of "Gaul chicken".
Another nickname used by many netizens in China to describe France is "Milk Law". This terrier originated from the performance of France in World War II. It surrendered before the battle and let the moustache steal the house at once. It roughly means "humiliating France". This nickname is actually a French nickname that our generation of netizens have more contact with. It's a mockery.