See http://saml.rilspace.org/node/248

gc_samuel.d

import std.stdio;
import std.string;
import std.algorithm;
import std.regex;

void main() {
	File file = File("Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa","r");
	int gcCount = 0;
    int atCount = 0;
	int totalBaseCount = 0;
    string line;
    char c;
	while (!file.eof()) {
		line = file.readln();
		if (line.length > 0 && !startsWith(line, ">")) {
            for(uint i = 0; i < line.length; i++) {
                c = line[i];
                if (c == 'G' || c == 'C') {
                    gcCount++;
                } else if ( c == 'A' || c == 'T' ) {
                    atCount++;
                }
                   
            }
        }
    }
	float gcFraction = ( cast(float)gcCount / cast(float)totalBaseCount );
    writeln( gcFraction * 100 );
}

Compile flags used:

dmd -ofgc_samuel -O -release -inline gc_samuel.d