Dear Filipe,
thanks for your answer.
Following your suggestion, I have tried the attached file.

The output is:
$ python /data/compress.py
b'68963e137ced6ee2aa5a93e155b201a3c172e2683d4b15a0eab7c1d8d43e48b4  /tmp/py_gzip.tgz\n'
b'68963e137ced6ee2aa5a93e155b201a3c172e2683d4b15a0eab7c1d8d43e48b4  /tmp/py_gzip.tgz\n'
$ rm -rf a/
$ mv py_gzip.tgz py_gzip.tgz0
$ python /data/compress.py
b'9c897d82c332f0d5443fe66112abe5f318bf6e6574e44c5c3c385f398784ac35  /tmp/py_gzip.tgz\n'
b'9c897d82c332f0d5443fe66112abe5f318bf6e6574e44c5c3c385f398784ac35  /tmp/py_gzip.tgz\n'
$ diffoscope py_gzip.tgz0 py_gzip.tgz
--- py_gzip.tgz0
+++ py_gzip.tgz
│   --- py_gzip.tgz0-content
├── +++ py_gzip.tgz-content
│ ├── file list
│ │ @@ -1,4 +1,4 @@
│ │ -drwxr-xr-x   0 root         (0) root         (0)        0 2021-05-30 15:32:56.566535 a/
│ │ --rw-r--r--   0 root         (0) root         (0)        6 2021-05-30 15:32:56.566535 a/eph0
│ │ --rw-r--r--   0 root         (0) root         (0)        6 2021-05-30 15:32:56.566535 a/eph1
│ │ --rw-r--r--   0 root         (0) root         (0)        6 2021-05-30 15:32:56.566535 a/eph2
│ │ +drwxr-xr-x   0 root         (0) root         (0)        0 2021-05-30 15:33:16.956535 a/
│ │ +-rw-r--r--   0 root         (0) root         (0)        6 2021-05-30 15:33:16.956535 a/eph0
│ │ +-rw-r--r--   0 root         (0) root         (0)        6 2021-05-30 15:33:16.956535 a/eph1
│ │ +-rw-r--r--   0 root         (0) root         (0)        6 2021-05-30 15:33:16.966535 a/eph2

Even if I am specifing an mtime, it is not correctly applied.

Thanks