If you're on a shared server, you will not be able to change the system clock or time zone. The best you can do is set your desired (GMT) cron event time to the server's time zone (PST/PDT for Lunarpages), provided you don't mind a one hour offset part of the year. If this is unacceptable (it absolutely has to be at the same GMT each day), you could trigger on both the hours (2 per day) and pick whichever one is the correct GMT (depending on whether DST is in effect). That would be a little less load on the server than running cron hourly.
VPS might be able to run on GMT. Dedicated servers probably can.