Gzran is a package that reads arbitrary offsets of uncompresssed data from compressed gzip files. This is accomplished by first inflating a file and saving decompressor state into memory. Then, the built index can be used with the Extract function to read from the compressed file without fully inflating the file.

Gzran is based on the c library, zran, by Mark Adler: https://github.com/madler/zlib/blob/master/examples/zran.c

idx, err := gzran.BuildIndex(gzippedFileName)
if err != nil {
    panic(err)
}
b, err := gzran.Extract(gzippedFileName, idx, fileOffset, readLength)
if err != nil && err != io.EOF {
    panic(err)
}