You can find the Stephanos entry here, with the original Greek on the left. It says simply that it was a city of Gaul, and that a "Parthenios" says it was named "after the Herikleidan Nemausos" (ἀπὸ Νεμαύσου Ἡρακλείδου). Parthenius was either a 1st century BCE Greek poet living in Rome (?), whose only surviving works are a collection of love stories, from Nicaea; or alternately a Parthenius "of Phocaea" who lived centuries later, in the 4th century CE. I tend to think it is the latter, and that this tradition of the naming of Nemausus is a very late story. Stephanos also gives alternates of Nemausios and Nemausinos, without comment, and I can't find anything at all for either of those.

Meanwhile: Nemausos is well-attested as the Celtic name of a local god of a spring of the local people around the town. There are numerous inscriptions at Nimes that mention this god "Nemausos." For instance:

Jovi et Nemauso Titus Flavius Hermes Exactor operum basilicae marmorarii et lapidarii votum solvunt ("Titus Flavius Hermes, the inspector of the construction of the basilica, and the marble-workers and stone-workers, fulfill their vow to Jupiter and Nemausos").

This was erected in Nimes in the early first century CE, during the reign of Augustus (when the colonia at Nemausus was established).