GraphQL doesn't transfer raw bytes, so they would save the 33% overhead if they weren't using Base64 binary-to-text encoding.
My thinking is that while in this case it's "probably safe" to use Gzip + HTTPS, but that's not a good practice to build secure systems. If e.g. here these requests are exempted and run to Gzip, the reasons and circumstances for doing it here will be forgotten, and can end up with other stuff exempted later which shouldn't. Just from experience how things work in large orgs.