There is. Some planes beep, others literally play a warning "too low, gear, too low, gear" in the cockpit and headsets.
But gear up landings don't happen in a normal stable approach in nice weather. Gear up landings happen when something else is going on taking all the pilots' attention. Like in this incident they were too high, too fast and descending too quickly. That causes a lot of other warnings and distractions, so they completely didn't hear the gear warning.
But gear up landings don't happen in a normal stable approach in nice weather. Gear up landings happen when something else is going on taking all the pilots' attention. Like in this incident they were too high, too fast and descending too quickly. That causes a lot of other warnings and distractions, so they completely didn't hear the gear warning.