 According to the United States Census Bureau, the township has a total area of , of which, of it is land and of it (0.13%) is water.Gregg Township is bordered by Lycoming County to the west and north, the West Branch Susquehanna River to the east, over which lies Northumberland County and White Deer Township to the south.As of the census of 2000, there were 4,687 people, 335 households, and 249 families residing in the township.