Which airport was the site where The Grand National horse race was run during WWI?

The Grand National is a National Hunt horse race held annually at Aintree Racecourse, near Liverpool, England. First run in 1839, it is a handicap steeplechase over an official distance of about 4 miles and 2½ furlongs, (or accurately 4 miles 514 yards (6.907 km)), with horses jumping 30 fences over two laps. It is the most valuable jump race in Europe, with a prize fund of £1 million in 2017. An event that is prominent in British culture, the race is popular amongst many people who do not normally watch or bet on horse racing at other times of the year.

For three years during the First World War, while Aintree Racecourse was taken over by the War Office, an alternative race was run at Gatwick Racecourse, a now disused course on land now occupied by Gatwick Airport. The first of these races, in 1916, was called the Racecourse Association Steeplechase, and in 1917 and 1918 the race was called the War National Steeplechase. The races at Gatwick are not always recognised as "Grand Nationals" and their results are often omitted from winners' lists.