Use header.Values for check header instead of header.Get

main.go

package main

import (
	"fmt"
	"net/http"
	"net/http/httptest"
	"strings"
)

const encodingHeader = "Accept-Encoding"

func isGzipByGet(header http.Header) bool {
	return strings.Contains(header.Get(encodingHeader), "gzip")
}

func isGzipByValues(header http.Header) bool {
	for _, v := range header.Values(encodingHeader) {
		if v == "gzip" {
			return true
		}
	}
	return false
}

func main() {
	server := httptest.NewServer(http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
		fmt.Println(request.URL)
		fmt.Println("values:", request.Header.Values(encodingHeader))
		fmt.Println("can gzip by get", isGzipByGet(request.Header))
		fmt.Println("can gzip by values", isGzipByValues(request.Header))
		fmt.Println()
	}))
	defer server.Close()

	req, _ := http.NewRequest(http.MethodGet, server.URL+"/with-gzip-first-place", nil)
	req.Header.Add(encodingHeader, "gzip")
	req.Header.Add(encodingHeader, "asd")
	_, _ = http.DefaultClient.Do(req)

	req, _ = http.NewRequest(http.MethodGet, server.URL+"/with-gzip-second-place", nil)
	req.Header.Add(encodingHeader, "asd")
	req.Header.Add(encodingHeader, "gzip")
	_, _ = http.DefaultClient.Do(req)

	req, _ = http.NewRequest(http.MethodGet, server.URL+"/without-gzip", nil)
	req.Header.Add(encodingHeader, "asd")
	_, _ = http.DefaultClient.Do(req)
}

Output:
/with-gzip-first-place
values: [gzip asd]
can gzip by get true
can gzip by values true

/with-gzip-second-place
values: [asd gzip]
can gzip by get false
can gzip by values true

/without-gzip
values: [asd]
can gzip by get false
can gzip by values false