Funny how it's kind of hard to find documentation on this fairly simple task. The host I'm working with is CentOS 5, a variant of Redhat Enterprise 5. I use the following commands:
cat /usr/share/zoneinfo/GMT > /etc/localtime
That is actually what sets the time. It's a binary file and is what our system tools and libraries look at.
Edit the ZONE to be GMT. Mine looks like this:
ZONE="GMT" UTC=true ARC=false
Verify it looks right with these commands.
date ; date -u ; /sbin/hwclock --show