You may only run gfs2_grow on a mounted filesystem; expansion of unmounted filesystems is not supported. You only need to run gfs2_grow on one node in the cluster. All the other nodes will see the expansion has occurred and automatically start to use the newly available space.
You must be superuser to execute gfs2_grow. The gfs2_grow tool tries to prevent you from corrupting your filesystem by checking as many of the likely problems as it can. When expanding a filesystem, only the last step of updating the resource index affects the currently mounted filesystem and so failure part way through the expansion process should leave your filesystem in its original unexpanded state.
You can run gfs2_grow with the -T flag to get a display of the current state of a mounted GFS2 filesystem.
The gfs2_grow tool uses the resource group (RG) size that was originally calculated when mkfs.gfs2 was done. This allows tools like fsck.gfs2 to better ensure the integrity of the file system. Since the new free space often does not lie on even boundaries based on that RG size, there may be some unused space on the device after gfs2_grow is run.
There is no way to shrink a GFS2 filesystem.