How can I use gzip on a string?

You can just write using gzip.Writer as it implements io.Writer.

Example:

package main

import (
    "bytes"
    "compress/gzip"
    "fmt"
    "log"
)

func main() {
    var b bytes.Buffer
    gz := gzip.NewWriter(&b)
    if _, err := gz.Write([]byte("YourDataHere")); err != nil {
        log.Fatal(err)
    }
    if err := gz.Close(); err != nil {
        log.Fatal(err)
    }
    fmt.Println(b.Bytes())
}

Go Playground

If you want to set the compression level (Default is -1 from compress/flate) you can use gzip.NewWriterLevel.