We have 3 GFS enabled clusters in my center. I love it, but it is daunting to set up.
We also use the 56F, and it is a great switch (It's just a Brocade Silkworm)
As to how to set it up, here is a link to the CentOS documentation (These steps will work for any redhat based system):
http://mirror.centos.org/centos/4/docs/
It's the the section called Cluster Suite - Global Filesystem.
Basically, your nodes will have to be clustered via Cluster Suite and set up with "fence" devices before you will be able to use GFS. The cluster uses fence to immediately cut access off to a FS from a node if the node fails. This keeps the file system consistent. Once a node fails, the other nodes look at the journal and see if there were any transactions that need to be rolled back or completed. This prevents corruption.
It is dificult to set up, but works extremely well once it's completed. We have about 20 TB of data stored this way across our 3 clusters, and the clusters serve 10 million page views per day from these file systems.
Brandon