A series of small buildings winding up the hillside comprises the active Buddhist monastary in Gangu, culminating in a large Budda built into the cliff face at the top. Don't miss the underground part, filled with wall paintings, reliefs and statues. When I was there in 2004 they were doing rather extensive additional digging.
A bus runs from Tianshui to Gangu roughly once an hour. The trip takes about an hour and a half (through some beautiful hills). Last bus back leaves around 5, but trains back to the Beidao side of Tianshui run later.