List the genres of Forrest Gump movie.
genres refers to genre_name; Forrest Gump movie refers to title = 'Forrest Gump'
SELECT T3.genre_name FROM movie AS T1 INNER JOIN movie_genres AS T2 ON T1.movie_id = T2.movie_id INNER JOIN genre AS T3 ON T2.genre_id = T3.genre_id WHERE T1.title = 'Forrest Gump'