This is because POSIX-style signs in the Zone names and the output abbreviations are reverse
*Setting can be found in Operating environment > Advanced > Timezone
When you set the timezone in gateway the setup file, it is equivalent to setting TZ environment variable in the command prompt and then using date to print the time in that timezone.
setenv TZ Europe/London; date // Assume date command shows output as Fri Sep 26 14:00:00 BST 2014
then, setenv TZ GMT; date // will show output as Fri Sep 26 13:00:00 BST 2014
and if you do setenv TZ GMT+1; date // This will give output Fri Sep 26 12:00:00 BST 2014 ??
and setenv TZ GMT-1; date // This will give output Fri Sep 26 14:00:00 BST 2014 ??
Confused about last 2 outputs? This is because POSIX-style signs in the Zone names and the output abbreviations are reverse of what many people expect.
So, when you say GMT+1, it means one hour behind GMT (i.e. west of Greenwich) //although our intuition says one hour ahead of GMT
And when you say GMT-1, it really means one hour ahead of GMT (i.e. east of Greenwich) //although our intuition says one hour behind of GMT
More info here: http://en.wikipedia.org/wiki/Tz_database
Gateway is just following the standard conventions. So, you have to use reverse signs to achieve the time you want to set.