When a client asks for a gzip-encoded consensus, the server sends zlib
$ curl -s -O --header "Accept-Encoding: gzip" 188.8.131.52:9030/tor/status-vote/current/consensus && file consensus Accept-Encoding: gzip Requested: status-vote/current/consensus consensus: zlib compressed data
But when the client asks for gzip-encoded descriptors, the server sends gzip:
Accept-Encoding: gzip Requested: server/authority authority: gzip compressed data, max compression, from Unix
See https://trac.torproject.org/projects/tor/ticket/25667#comment:11 for a full list of requested encodings, documents, and served formats.
I think this is a minor bug with no impact, because tor clients will decompress the zlib anyway. (I don't know if tor clients ever ask for gzip without zlib.)
But I'd like someone who knows the compression code better to confirm.