On a storage node, GlusterFS bricks (the physical disks composing a GlusterFS volume) are typically mounted at a specific location. For example: /export/glusterfs/<volume>/brick0/brick.
However to use a GlusterFS volume, it must be mounted:
mount -t glusterfs gluster.example.com:/<volume> /mnt
Now I can change all the files in /mnt and GlusterFS will do its magic. However, we can also see the same files in /export/glusterfs/<volume>/brick0/brick.
Can the files in /export/glusterfs/<volume>/brick0/brick be changed without breaking GlusterFS?
Yes, technically you can modify files in the underling volume, but Gluster will not be notified about your changes, and therefore they may not be replicated to other Gluster nodes.
This is very much not recommended, and could mean that your servers end up with different underlying files, which can lead to unpredictable behaviour/data loss.