Gaza is a dense area for such high explosives. I am not even sure Israel has such weapons. As it is, civilian toll is causing international protests. MOAB has been used only once in Afghanistan in the mountain areas.Israel can use MOAB, they don't need to pinpoint exact locations, they just need to throw in suspect areas and let MOAB handle the rest.
Even if MOAB cannot destroy the tunnel, the pressure from the explosion and shock wave can destroy the exits and seal the tunnel door, leaving those inside trapped forever.