Gallant was transferred from the Mediterranean Fleet shortly after the beginning of World War II to the British Isles, to escort shipping in local waters.

Gallant carried a maximum of 470 long tons (480 t) of fuel oil that gave her a range of 5,530 nautical miles (10,240 km; 6,360 mi) at 15 knots (28 km/h; 17 mph).

[5] When World War II began in September 1939, Gallant was in the Mediterranean, but she and her entire flotilla were transferred to the Western Approaches Command at Plymouth in October.

On 2 February 1940 Gallant and her sister ship, Griffin, rescued the crew from the oil tanker British Councillor which was sinking after it had struck a mine.

Gallant took over escorting Convoy HN 12 after the destroyer Duchess was sunk on 18 February and she rescued 12 survivors from the Swedish ship Santos near Duncansby Head a week later.

[6] While Gallant was participating in the Dunkirk evacuation, a near miss by a bomb on 29 May knocked out her steering and caused minor damage to her hull and electrical systems.

[6] During Operation Hurry, Gallant, and three other destroyers, escorted Argus to a position south-west of Sardinia so the carrier could fly off her Hurricanes to Malta on 2 August.

The ship escorted the battleship Barham and the cruisers Berwick and Glasgow during Operation Coat in early November as they joined the Mediterranean Fleet.

[6] On 10 January 1941, during Operation Excess, the Italian torpedo boats Circe and Vega attempted an attack on the Allied convoy off Pantellaria.

Right after the engagement, in which Vega was sunk, Gallant struck a mine that detonated her forward magazine, because the Italian action pushed the British convoy too much south of their pre-established route.