See http://saml.rilspace.org/moar-languagez-gc-content-in-python-d-fpc-c-and-c

gc.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strings"
)

func main() {
	var gc int = 0
	var at int = 0
	var line string
	var err error
	file, err := os.Open("Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa")
	var fileReader *bufio.Reader = bufio.NewReader(file)
	for err == nil {
		line, err = fileReader.ReadString('\n')
		if !strings.HasPrefix(line, ">") {
			var curChar byte
			for i := 0; i < len(line); i++ {
				curChar = line[i]
				if curChar == 'G' || curChar == 'C' {
					gc++
				} else if curChar == 'T' || curChar == 'A' {
					at++
				}
			}
		}
	}
	var gcFraction float32 = float32(gc) / float32(at+gc)
	fmt.Println(gcFraction * 100)
}