In short: gazans are all issued ID numbers at birth. The ministry of health has published id numbers of the dead, which means you can do stats and tell if the data is fake. On top of that, so far (afaik since 2009 when hamas came to power) nobody has caught them in a lie. So they’ve a track record of telling the truth, and give us data that we can smell-test for fraud and it passes.
So yes, nobody else is on the ground to produce independent numbers, so the numbers can’t be fully verified. But using that doubt as an excuse for inaction in the face of ethnic cleansing and genocide is fucking disgusting.