Is it possible to tweak AES-GCM so that it is satisfactory for whole-disk encryption (like XTS mode)?
The ZFS file system uses AES in CCM or GCM modes.  This works because in ZFS the data and file system metadata is encrypted but the block pointers are in the clear, the AuthTag (MAC) is stored in the block pointer.  ZFS also has a SHA256 based merkle tree based on the block pointers that is used for data integrity for resilvering and navigation purposes.

Note that ZFS is file system (actually object based really) encryption it does not claim to be "full disk encryption".

For more details see here: https://blogs.oracle.com/darren/entry/zfs_encryption_what_is_on

Darren J Moffat (Architect and lead developer for ZFS encryption).