When a pilot forgets to extend the gear when performing a landing, the aircraft should warn the pilot of this. Usually there is an aural warning when the throttles are below a certain power setting and the gear is not down. With commercial transport aircraft this warning can be given with certain flap settings or radio altitudes.

CS 25.729 Extending and retracting mechanisms
(e)(2) The flight crew must be given an aural warning that functions continuously, or is periodically repeated, if a landing is attempted when the landing gear is not locked down.
(3) The warning must be given in sufficient time to allow the landing gear to be locked down or a go-around to be made.

There is a quite close competitor to the correct answer, that mentions inadvertent retraction of gear on the ground. As far as we can tell, an aural warning in this scenario is not a legal requirement, but a warning of some kind IS a requirement, due to the following regulation, and the fact that gear must not retract when on the ground, due to weight-on-wheels logic switches, mechanical downlocks, etc.

CS 25.729 Extending and retracting mechanisms
(e)(7) A clear indication or warning must be provided whenever the landing gear position is not consistent with the landing gear selector lever position.