It's true, using some rather non-obvious trickery: https://github.com/madler/zlib/blob/develop/examples/zran.c

I also wrote a tool to make a randomly modifiable gzipped disk image: https://rwmj.wordpress.com/2022/12/01/creating-a-modifiable-...

Sure, but zstd soundly beats gzip on every single metric except ubiquity, it is just straight up a better compression/decompression strategy.