Time zone 'GMT+8'

The target pgsql is compiled from 8.4rc1. 'GMT+8' can be found in installed binary file .../share/postgresql/timezone/Etc/GMT+8.

This is the recorded script:

-----BEGIN record----------
db1=# select now();
now
-------------------------------
2009-11-29 14:44:37.322414+08
(1 row)

db1=# set timezone to 'GMT+8';
SET
db1=# select now();
now
-------------------------------
2009-11-28 22:45:03.397545-08
(1 row)

db1=# set timezone to GMT-8';
SET
db1=# select now();
now
-------------------------------
2009-11-29 14:45:39.160701+08
(1 row)

db1=# set timezone to '0';
SET
db1=# select now();
now
-------------------------------
2009-11-29 06:45:54.347482+00
(1 row)
-----END record----------

I thought time zone 'GMT+8' was '8' or UTC+8, and 'GMT-8' was '-8' or UTC-8. Does the original time zone settings shipped with source have special interpretation that is different from my understanding?

Regards,
CN