A fair question, but it is really not difficult to see why the claim must be true. Here [1] is one source for the death toll in Gaza by direct military action (not including people burried under the rubble of destroyed buildings, dying of starvation, all the lives that disease will claim over the following weeks and months and so on).

About 70% of killed people are estimated to be women and children. Those are not Hamas fighters. Even if you assume that every adult male that has been killed was a Hamas fighter (which is obviously not going to be the case), and even if we pretend that men being killed is okay, that is still 70% of civilians dying compared to enemy combatants. That is the basis for "overwhelmingly civilian" deaths claim.

Israel also constantly claimed that Hamas is hiding in tunnels underground, yet they were killing people by bombing residential buildings in a display of logical contradiction. That alone will lead one to conclude that they are mostly killing civilians.

[1] https://www.reuters.com/world/middle-east/how-many-palestini...

> 20,000 overwhelmingly civilian deaths

Just putting it out there that the median age in Palestine is 18. Almost half the population is children. Figure out the rest yourself.