Create PK on existing Table

ALTER TABLE build_test ADD COLUMN g_id SERIAL;
UPDATE build_test SET g_id = nextval(pg_get_serial_sequence('build_test','g_id'));
ALTER TABLE build_test ADD PRIMARY KEY (g_id);

or

ALTER TABLE build_test ADD COLUMN g_id SERIAL;
UPDATE build_test SET g_id = DEFAULT;
ALTER TABLE build_test ADD PRIMARY KEY (g_id);