A gzip file can have uncompressed data written to it, writing compressed data to the underlying file. It can also have uncompressed data read from it, reading compressed data from the underlying file.

However, it does not support reading compressed data from an underlying uncompressed file, nor writing compressed data to an underlying uncompressed file.

This makes it impossible to, for example, obtain an arbitrary file-like object and produce another file-like object which transparently compresses data read from the first.