For me gravity sync was too heavy and cumbersome. It always failed at copying over the gravity sqlite3 db file consistently because of my slow rpi2 and sd card, a known issue apparently.
I wrote my own script to keep the most important things for me in sync: the DHCP leases, DHCP reservations and local DNS records and CNAMES. It’s basically just rsync-ing a couple of files. As for the blocklists: I just manually keep them the same on both piholes, but that’s not a big deal because it’s mostly static information. My major concern was the pihole bringing DHCP and DNS resolution down on my network if it should fail.
Now with keepalived and my sync script that I run hourly, I can just reboot or temporarily shutdown pihole1 and then pihole2 automatically takes over DNS duties until pihole1 is back. DHCP failover still has to be done manually, but it’s just a matter of ticking the box to enable the server on pihole2, and all the leases and reservations will be carried over.