To debunk denialism of the Gaza death toll, all you need to do is point out that when we get them, the IDF casualty estimates very closely track with Gaza Health Ministry estimates. Since both of the opposing parties roughly agree on the death toll, it is likely a fairly accurate figure.
Official source? I haven't checked in a long time, but it's been awhile since I've seen any official source call the Health Ministry numbers into question. Back when they were around 30k or so, it was largely agreed on by the IDF.