Is DRBD the only viable block replication solution for Linux?
Yes, DRBD is the only replicated block device out there that can handle concurrent writes. If you plan to put a filesystem on top, it obviously needs to handle multiple writers as well, like GFS(2) and OCFS(2) do.
Please note that if you can afford higher levels of abstraction for redundancy, you'll likely be much, much happier with file-level semantics, so you should really think twice before going with block-level semantics. If you can't use higher levels of abstractions, but have money to throw at the problem, you can achieve significantly better performance with a good SAN.
But you probably know that already.
Well, there's also MARS (Light). According to the documentation this widely used at German provider 1&1
You could set up a RAID set using iSCSI devices but I'd be wary of doing this with asymmetric storage devices (which in the case of remote storage includes the network) - OTOH DRBD is explicitly designed to support such usage.
Is there a reason you don't like DRBD?
Have I failed in my research
If you think that GFS is a replicating cluster filesystem, then I'm afraid so.