Backup works like any other system; you do it app layer. In high-volume high-sensitivity apps, the people running the disk server also back it up, by running protocols that mirror block-level changes to a second rack full of disks somewhere in Tennessee.
(yes, yes, or IDE or FC, yes yes, or software initiator, etc etc).
GFS/HDFS are officially called filesystems, but they're about as much as a filesystem as a Samba server: they expose files to the outside world, with some extra layers of abstraction you can mount the filesystems under Linux, but in essence they live on top of existing filesystems themselves and have nothing in common with the traditional filesystems, where you operate on a block device. A SAN, otoh, is such a block device and traditional filesystems operate on them.
This means that you can run a RDBMS on a SAN, while you can't on GFS/HDFS. This covers a common use case where you have a central expensive database server, with dozens of terrabytes of storage in a SAN, which you see in a lot of large corporations.