Thank you...

gzip is a block compressor. You don't have to have access to the entire file to recover some of the data. You can actually seek to the beginning of a gzip block and start reading without having read all the data that came before.

A lot of media formats have this property as well-- mp3 for one. If the beginning of the mp3 is lost, you can still access the last part. Video formats often have 'key frames' that can be used to recover synchronization if the decoding got screwed up.

Being able to recover something, even if part of the file is messed up, is a very important property for log files. Disks can and do fail and start returning garbage-- I just had one turn this week. I hope that the journald designers think about issues like this, at least in later versions of the format.