I'm not sure what you are asking here. The gods of the Romans (Jupiter, Mercury, etc) are considered to be among the larger group called "pagan" or "pre-Christian." Are you asking about Roman syncretism with other pantheons in the empire, like the Greeks, Gauls, Egyptians, etc.?

If I'm interpreting your question correctly (based on your other post about pagan gods on the Isle of Wight), "pagan" means different things in different contexts.

Pagan is a Latin word, from "pāgānus" meaning "rustic, of the countryside" and related to the English word "peasant". In a Roman context, the pagan gods were the old Greco-Roman pantheon, or by extension, any polytheistic belief system.

By the later Roman empire, Christianity and other religions had largely replaced the traditional religion for urban Romans. "Pāgānus" was also military slang for a civilian, so Christians might have used it to refer to anybody who wasn't part of the new elite.

Whichever meaning was intended, "pagan" was meant as a slight against anyone who was still old-fashioned enough to be polytheistic, and did not specifically refer to the Norse/Germanic gods you reference in your other post.