What happens when one of the storage bricks goes down (HDD failure), how does GlusterFS deal with data recover? Is the stored data still safe?
Gluster has built in data "translators" that automatically replicate data across all of your bricks. The particular type of translator you're interested in is called AFR for automatic file replication. The AFR translator also uses the DHT (distributed hash table) translator. It's important that you have at least two master bricks since if you only have one, you have a single point of failure. As long as you have at least two bricks, an auto-healing process is triggered by GlusterFS's daemon (it's automatically installed when you invoke
gluster the first time) using the other master server's replicated/translated data if anything goes wrong. Actual disaster recovery that the auto healing can't fix requires going through a careful process documented here.
Other wisdom on the topic (for example, Redhat's documentation) recommends having at least six bricks in two sets, actually. This way, "even if we lose two bricks from each set, there is no data loss". Basically, the more bricks you have, the more redundancy you have, and the less chance of data loss you have (however, it'll become cost-prohibitive and time-prohibitive after a while).
Usually you have at least two replicas of data between the nodes of GlusteFS cluster.
If you need details about handover between the nodes (in case of one node fail) you can check this answer in Unix and Linux SE site