Short answers, pulled from the story.
Gallo-Roman religion was a fusion of traditional Gaulish religious practices with Roman and Hellenistic religions introduced during Roman Imperial rule. Scholars describe it as the product of selective acculturation, meaning each side absorbed elements from the other rather than one simply replacing the other.
Epona was a native Celtic equine goddess who was adopted into Roman religious practice across the empire. She was unusual because most Celtic deities either merged with Roman counterparts or remained local; Epona crossed over into Roman worship more completely than almost any other Gaulish deity.
Cernunnos, an antlered deity typically depicted sitting cross-legged, was considered too distinct from any figure in the Roman pantheon to be synchronized. Roman writers compared him to Mercury, Jupiter, Actaeon, and Dis Pater, but no interpretatio Romana took hold for him.
Emperor Augustus banned the druids. His successor Tiberius extended those restrictions to vates and healers. Gallo-Roman priests adopted Roman models of religious leadership in their place, and Drusus, the son of Tiberius, established a cult center near Lyon.
Gallo-Roman temples, known as fana, were typically concentric in shape and often quadrangular. Unlike classical Roman temples, their entrances usually faced east. Many were built near water sources, reflecting Celtic attitudes toward water. They often retained the layouts of earlier Celtic sites while adopting Roman building materials.
Emperor Claudius prohibited human sacrifice, but the practice may already have been declining before that law. Archaeological evidence from Belgic Gaul suggests it may have continued. Headhunting similarly persisted; artwork from Trajan's Column and reliefs from Paris and Arles depict severed heads in military and votive contexts, suggesting the practice continued within the Roman military itself.