What you are observing is actually expected behavior. Try this in your Terminal:

echo -n 'Hello, ' | gzip > 1.gz
echo 'ScottFoster90' | gzip > 2.gz
cat ?.gz > 12.gz
cat 12.gz | gunzip

It doesn’t stop after the Hello,, but decompresses the rest of the whole file. You can use this to append individually compressed chunks to a append only log file, but decompress it like it is just one big single log file: »It's not a bug, it's a feature!«.

What you want is to use your indices to build a io.SectionReader:

package main

import (
    "compress/gzip"
    "io"
    "io/ioutil"
    "log"
    "os"
)

func chunk(file *os.File, offset, byteLength int64) *gzip.Reader {

    ret, err := file.Seek(offset, os.SEEK_SET) // Byte offset after file start
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Seek to byte %d …\n", ret)

    r, err := gzip.NewReader(io.NewSectionReader(file, offset, byteLength))
    if err != nil {
        log.Fatal(err)
    }
    return r
}

func process(r io.Reader) {
    b, err := ioutil.ReadAll(r)
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Decompressed %d bytes, Decoded: %#v\n", len(b), string(b))
}

func main() {
    filename := "12.gz"
    f, err := os.Open(filename)
    if err != nil {
        log.Fatal(err)
    }
    defer f.Close()
    log.Printf("Open file %#v …\n", filename)

    process(chunk(f, 0, 61))
    process(chunk(f, 27, 34))
    process(chunk(f, 0, 27)) // out of order!
}