Google has on top of ext4 his GoogleFS, in theory you could even go 'without' a underlying FS, a bit like BlueStore from Ceph.
This does not sound right to me.
If you're talking about GFS, that's been gone for a decade, and was not a filesystem as far as the kernel was concerned.
That's why i brought up the Ceph example, but counts for databases etc too, actually the early databases had all their own fs/data-structure on the platter.