gtlions=# alter user gtlions password 'gtlions';
ALTER ROLE
gtlions=# select * from pg_shadow;
 usename | usesysid | usecreatedb | usesuper | usecatupd | userepl |               passwd                | valuntil | useconfig
---------+----------+-------------+----------+-----------+---------+-------------------------------------+----------+-----------
 gtlions |       10 | t           | t        | t         | t       | md56c7d333ce7a7f5882bdc5a232c107d81 |          |
(1 row)

gtlions=# \q

gtlions:~ gtlions$ python
Python 2.7.6 (default, Sep  9 2014, 15:04:36)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import hashlib
>>> password = 'md5' + hashlib.md5("gtlionsgtlions").hexdigest()
>>> password
'md56c7d333ce7a7f5882bdc5a232c107d81'

about gpdb password md5