There are two ways within systemd to manage the fact that you need the gluster service active before mounting the mountpoint. Thus in the past the mount triggered before the gluster service was running and didn't mount again.
/etc/fstab
You can use the x-systemd.autmount feature which mounts the mountpoint as soon as it's accessed
localhost:/shared /gluster/shared glusterfs defaults,_netdev,noauto,x-systemd.automount 0 0
Unit file
This unit file takes care of the mountpoint and makes sure gluster is running before:
[Unit]
Description=GlusterFS Mount
After=glusterfs-server.service
Requires=glusterfs-server.service
[Mount]
What=localhost:/shared
Where=/gluster/shared
Type=glusterfs
Options=defaults,_netdev
[Install]
WantedBy=multi-user.target
You should put it in /etc/systemd/system/gluster-shared.mount and then enable it:
systemctl enable gluster-shared.mount