how to go from drum tower railway station to gulou campus of nanjing university

The drum tower railway station? I think you may mean the Drum Tower Subway Station?
If you mean the subway station, there are two methods for you. The first method is to get out of the station from Exit 2 and walk along West Beijing Road to pass the Drum Tower Garden to reach Tianjin Road. Then walk along the Tianjin Road to that university.
The second method is to get out of the station from Exit 1 and walk along the Zhongshan Road to Hankou Road. Then turn to walk along the Hankou Road to that university easily.