CREATE TABLE city (city_id DECIMAL, city TEXT, hanzi TEXT, hanyu_pinyin TEXT, regional_population DECIMAL, gdp DECIMAL)
CREATE TABLE match (match_id DECIMAL, date TEXT, venue TEXT, score TEXT, result TEXT, competition TEXT)
CREATE TABLE hosting_city (year DECIMAL, match_id DECIMAL, host_city TEXT)
CREATE TABLE temperature (city_id DECIMAL, jan DECIMAL, feb DECIMAL, mar DECIMAL, apr DECIMAL, jun DECIMAL, jul DECIMAL, aug DECIMAL, sep DECIMAL, oct DECIMAL, nov DECIMAL, dec DECIMAL)

what is the GDP of the city with the largest population.
SELECT gdp FROM city ORDER BY regional_population DESC LIMIT 1
SELECT "gdp" FROM "city" ORDER BY "regional_population" DESC NULLS LAST LIMIT 1