Goldstone cites much more than only Hamas data. Among others physical evidence, movies, pictures, the IDF report, Human rights groups, Israeli soldiers, family of victimes, Israeli politicians, Israeli civillians, Israeli newspapers (like Haaretz). You will find that the IDF report is cited extensively. There is of course the fact that Palestinian cevillians were interviewed. As Goldstone points out there stories cross checked. When you call all Palestinians Hamas, that up to you. I think it is a one sided view.
Protests in southern Syrian city after Druze cleric's death (AP)
from the article: UN rights body ready to debate Goldstone probe