package main

import (
	"fmt"
	"io/ioutil"
	"log"

	"golang.org/x/image/font/sfnt"
)

func main() {
	b, err := ioutil.ReadFile("texgyrepagella-bold.otf")
	if err != nil {
		log.Fatal(err)
	}

	f, err := sfnt.Parse(b)
	if err != nil {
		log.Fatal(err)
	}

	var buffer sfnt.Buffer
	g, err := f.GlyphIndex(&buffer, '!')
	if err != nil {
		log.Fatal(err)
	}

	name, err := f.GlyphName(&buffer, g)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("The glyph name is %q.\n", name)
}

The glyph name is "exclam".

The glyph name is "".