'Gambit' was a British undertaking to station midget submarines as navigation beacons to guide landing craft involved in 'Neptune' (iii) and 'Overlord' (6 June 1944).

These X-craft marked the extreme eastern and western limits of the British and Canadian invasion beaches with navigation lights and flags off the 'Sword' and 'Juno' Beaches. X-20 and X-23 arrived in position on 4 June and, as a result of the delay imposed by bad weather on the start of 'Overlord', remained in position until 04.30 on 6 June (D-Day), when they surfaced and started using their navigational aids, which comprised an 18-ft (5.5-m) telescopic mast with a light shining to seaward, a radio beacon and an echo sounder tapping out a message for the minesweepers approaching the beaches.

The Royal Navy had offered a similar support to the US assault forces, but the offer was declined.