CREATE EXTENSION pg_trgm;
CREATE TABLE t (id int, s text);
INSERT INTO t SELECT g, 'item ' || g || ' ' || left(md5(g::text), 8) FROM generate_series(1, 10000) g;
CREATE INDEX t_s_gist ON t USING gist (s gist_trgm_ops);
SELECT attstorage FROM pg_attribute WHERE attrelid = 't_s_gist'::regclass;   -- p
ALTER TABLE t ALTER COLUMN s SET STORAGE EXTENDED;                          -- or MAIN
SELECT attstorage FROM pg_attribute WHERE attrelid = 't_s_gist'::regclass;   -- x
INSERT INTO t SELECT g, 'new item ' || g FROM generate_series(10001, 11000) g;