What is the ID of the production company which produced the movie "Gladiator"?
ID of the production company refers to company_id; movie "Gladiator" refers to title = 'Gladiator'
SELECT T2.company_id FROM movie AS T1 INNER JOIN movie_company AS T2 ON T1.movie_id = T2.movie_id WHERE T1.title = 'Gladiator'