When it came to the ancient Gauls, it’s important to note they were not a distinct, homogeneous culture but consisted of many tribes and influences from many places at different times. They were located in northeastern Spain, Turkey, southern Britain, northern Italy, France, southern and western Germany, Belgium, Luxembourg, and Switzerland.

During their travels as warriors, merchants, etc., and due to cultures coming into their areas, they would also be influenced by other cultures. Those cultures brought with them dêuoi and dêuâs, art styles, lifeways, and worldviews. This was not always a peaceful thing, but also not always a heated thing. Once introduced, such cultural elements formed an inherent part of the cultural identities of Gaul.

We find cultural mixing along the border areas, for example between the Gaulish tribes and the Germanic tribes along the Rhine. There was also peaceful trade with both Greeks and Romans before the Gallic Wars. Wine from the Græco-Roman world was a particularly prized commodity in Gaul. We will get into each in more depth below.

In many places where different peoples interacted, various forms of syncretism emerged.

Syncretism as defined by Merriam-Webster

The combination of different forms of belief or practice.
The fusion of two or more originally different inflectional forms.

So basically, syncretism is the blending of worldviews or cultural ideas to varying degrees. There are multiple views on the “hows” of syncretism.

In some cases, new identities were born of cultural mixing (cf. the Celto-Ligurians of modern-day Provence, the Celtiberians of northern Spain, or the Gallo-Greeks of Anatolia). In other cases, there might be more selective adoption and adaptation of elements from other cultures; for example, imported wine was essential to the feasting-hall culture of pre-Roman Gaul, quite distinct from the symposium culture of the Græco-Roman world.

In the religious domain, a common syncretistic concept is that one plus one equals three: in other words, a syncretic deity (e.g. Apollo Grannus) constitutes a sum greater than the parts (in this case, Apollo and Grannus), which may also remain distinct.

Gallo-Germanic – We find the Gauls located on the north and northeast borders of Gaul to have had significant cultural and material exchange with the neighboring Germanic tribes. Hotbeds of cultural and linguistic exchange were to be found in modern-day Switzerland, Belgium, the Netherlands, and parts of Germany (between the Rhine and Danube rivers). The neighboring Germanic and Gaulish tribes shared many of the same values, such as honor, family & tribal prosperity, hospitality, and honoring the gods. This and a common understanding of polytheistic/animistic framework and cycles of the world lent itself as an easy bridge for cultural exchange.
The Gauls and Germani shared worship sites and rite structures and often revered the same deities, sometimes under different names and sometimes under the same.
Some of the tribes who lived in lowlands and the Rhine-Alpine region were Germanic language–speaking Gauls. Some were Celticized Germani. Others even represented a mixed heritage due to generations of trading and inter marriage.

Gallo-Roman – Gallo-Roman polytheism refers to the religion of the period when Gaul was part of the Roman Empire. It is characterized by a fusion between Roman and indigenous religious elements. Celtic deities continued to be worshipped as new influences came in; the latter included the worship of Lares at the household shrine or lararium, the use of Latin, and the increasing use of stone for religious inscriptions and sculptures. Deities such as Venus, Juno, and Fortuna came to be widely worshipped in Gaul. Characteristic of the period are pairings of certain Roman and indigenous deities, such as Mercury and Rosmerta or Apollo and Sirona; the syncretism or identification of Roman and indigenous deities with each other, such as Mars Camulus or Sucellus Silvanus; and the use of Latin names for apparently local deities, such as Apollo for Apollo Grannus or Mercurius for Mercurius Visucius. The religious landscape changed as well; some pre-Roman religious sites were largely abandoned, while others continued to flourish, and new ones were established, particularly in newly founded cities or along the Roman road network. Stone temples were built in a distinctive Romano-Celtic style, the fanum (consisting of a cella surrounded by a wrap-around porch), while a unique religious monument—the Jupiter-giant column—emerged in the Rhineland and spread far beyond. Conversely, the worship of deities from Gaul was extended beyond Gaul via Roman military service or civilian settlement; Epona and the Matronæ were goddesses who gained particular prominence in this way.

Gallo-Hellenic – Greeks from Phocæa were early colonists of Massilia (present-day Marseille) in southern Gaul; from their interactions with the local Ligurian and Celtic populations was born Gallo-Greek polytheism. This includes not only Greek religion in Massilia and its colonies, such as Monœcus (Monaco) and Nicæa (Nice), but also Greek influences on neighbouring peoples. We see, for example, Celto-Ligurians laying out their cities after Greek models or participating in Greek-style hero cultus. The Gaulish language was written in the Greek alphabet in many inscriptions. Furthermore, Greek influence extended far beyond present-day Provence through pre-Roman trade links and later via Romanization (Roman high culture being bilingual and heavily influenced by Greek culture).
A different ‘Gallo-Greek’ fusion is to be found among the Galatians of west-central Anatolia (present-day Turkey around Ankara), where Gallic invaders settled in the 3rd century BCE. The Galatians preserved elements of their Celtic language and culture while adopting many characteristics of their new home, including the use of the Greek language for commerce, administration, and high culture, as well as the worship of deities such as Zeus (often in syncretized forms such as Zeus Bussurigios) and the Great Mother of the Gods (Cybele).

Gallo-Brythonic – Trade has connected people across what is now the English Channel for the vast majority of human history. We know that the Northern Gauls had allies, settlements, and a rich trade in Britain, as well as many shared dêuoi with their neighbours such as Taranis, Rosmertâ, and Camulos, to name only a few. Though differences certainly existed between the Gaulish and Brythonic (or Brittonic) cultures, they were both made up of a diverse patchwork of tribes that intersected in meaningful ways, often more connected by the body of water they shared than divided by it. With a worldview founded in Gaulish reconstruction, we also see strong influences from Breton and Welsh mythology, inscriptions in Roman Britain, and from shared values that we can piece together from proof of cooperation across the Channel.

Contributors: Viducus Brigantici filius, Nertatis Cingetos, Renōturos Epomapos, Satâ Dubniâs, Drunertos Tarvocnos