Not an aviation expert or enthusiast, but I'd imagine in a commercial airliner if the gear was not deployed and the pilot was trying to land, at a certain point the plane would start yelling at the crew something like "NO GEAR" "NO GEAR" "NO GEAR (deployed)"
So I don't think a pilot can just "forget" to deploy the landing gear in a commercial airliner.
So I don't think a pilot can just "forget" to deploy the landing gear in a commercial airliner.