When does InnoDB acquire gap locks on the post gap of the selected/updated records?

Question

I am currently learning about Gap Lock in the InnoDB engine, In MySQL doc they say, InnoDB will acquire a gap locks only on the preceding gap, but on this Percona article as I understood from their example, InnoDB will acquire gap locks on the pre and post gaps!

I tried to do my tests

CREATE TABLE T ( INT ID PRIMARY KEY);

INSERT INTO T 
VALUES (1), (2), (4), (7), (9);

//session-1
BEGIN;

//session-2
BEGIN;

//session-1
SELECT ID FROM T WHERE ID BETWEEN 4 and 7 FOR UPDATE;

//session-2 
INSERT INTO T VALUES(3);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

INSERT INTO T VALUES(8);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

//session-1 
commit;

//session-2
INSERT INTO T VALUES(8);
Query OK, 1 row affected (0.000 sec)

commit;

So the result is the same as the result described in Percona article! Is MySQL doc wrong or i just missed something? I really can't understand.

P.S: the Isolation-level for both sessions is the default one "REPATABLE READ".