gzip stores a timestamp by default in its header.
This prevents build to be reproducible and capture an uninteresting details.
Packages known to be affected:
file will display the timestamp if there's one.
strip-nondeterminism will take care of normalizing gzip'ed files.
Pass -n or --no-name option to gzip.
Quoting the manpage:
The environment variable GZIP can hold a set of default options for gzip.
It can be used for applications like tar.
Lintian tag: package-contains-timestamped-gzip
Ideally, we would set it as a default environment variable at the dpkg-buildpackage level. Unfortunately, this was experimented previously with CFLAGS (and the likes) similarily exported and this made many maintainers grumpy. *sigh*