After you have created your filesystem, you may add it to /etc/fstab:

/dev/vg-name/lv-name mountpoint gfs defaults 0 0

Do not forget to make this change on both cluster nodes.

After this, you may mount your new filesystem by starting the gfs service (on both nodes):

From then onwards, as long as you have DRBD configured to start automatically on system startup, before the RHCS services and the gfs service, you will be able to use this GFS file system as you would use one that is configured on traditional shared storage.