Historically, it was common for compressors to have symmetric decompressors, for one simple reason: when you start distributing files compressed with a new compression program, you want recipients to be able to easily decompress them. Generally speaking, when they're separate, the decompression program is simpler and smaller than the compression program, which makes it easier to transfer (think of transmission speeds and storage capacity thirty years ago), and easier to port to new systems too.
I can't find the original releases of
gzip so I don't know if that applied in this particular case; the oldest version I've found, 1.2.4, released in 1993, already used a single binary for
gunzip. Note that current versions still support this;
gunzip is just a wrapper shell script.