You don’t even need to pump it.
Most of Gaza is below sea level. Certainly the tunnels are.
Eqypt actually did this.
Just flood the whole damn area.
You have to pump it over the seawalls and barriers to start, but I’m also assuming that Hamas might have read everyone’s field manuals and is including grenade sumps, mantraps and other features that might keep low pressure water out. Pump it in at high pressure and watch for geysers to figure out where all the entrances were. Makes it easier to track down supporters that might have fled and give them ‘special’ attention.