One of the major sources that the Goldstone Commission heavily relied on in testimonies was TAWTHEQ. TAWTHEQ is quoted in the context of the Gaza war more often than HRW and seems to be mentioned in the report body about as often as PCHR (although PCHR is mentioned more in footnotes.) In other words, it seems that Goldstone put a lot of emphasis on TAWTHEQ's direct testimony to the Commission.
Who is TAWTHEQ? It is not an NGO - it is Hamas!
The organization is an Arabic acronym for the "Central Commission for Documentation and Pursuit of Israeli War Criminals." The possible objectivity of such an organization seems remote. Its webpage describes it as
It was established by the Gaza Ministry of Justice, which is Hamas.
TAWTHEQ " The Central Commission for documentation and pursuit of Israeli war criminals" is an independent governmental organization dedicated to monitoring and documenting the Israeli aggression crimes that targeted individuals, buildings, private and public property by the Israeli occupation forces, and particularly crimes that occurred during the last aggression on the Gaza Strip on 27 / 12/2008 until 19/1/2009 and pursuit war criminals and follow-up submitting lawsuits against them via the national and the international courts without any political affiliation to any party.
TWATHEQ [sic] is considered the only official commission to deal with the files of documenting aggressions and criminal acts of the Israeli aggression on Gaza, and follow-up submitting lawsuits against the Israeli officials.
While there are serious problems with relying on biased NGOs like PCHR (who refuses to call the Israeli army the IDF, instead using the term IOF consistently,) TAWTHEQ is single-mindedly created by Hamas not to be objective at all. Its mandate isn't to determine the existence of war crimes - it is to maximize the number of such "crimes."
So, for example, Goldstone quotes TAWTHEQ as saying that "341 of those killed were children. " It also quotes the PCHR and B'Tselem (314 and 320, respectively.) It is probable that TAWTHEQ (as well as DCI) is including children who were combatants in their list of victims.
The entire purpose of TAWTHEQ is to accuse Israel of war crimes and provide "evidence" to credulous NGOs. The fact that Goldstone gave it as much weight as it did seems to be a reflection of Goldstone's general bias towards those who testified in person, something that appears to be consistent throughout the report. As in the case of Addameer, Goldstone seems far less likely to question Gazan facts and figures than Israel's, even when the Arab organizations themselves have an explicitly anti-Israel mandate.