How long by taxi from Nanjing South Station to Gulou campus of NJ University?

It's about CNY 35.

You can take the taxi or the subway. Sometimes it is faster to take the subway as the taxi queue can be long, and traffic bad - the route by vehicle is more circular, whereas the subway is more direct. I'd say about 15 minutes, but depends on traffic.

The campus is less than 10 minutes walk from the Gulou subway station.