OS X Capitan [Error: Command failed: execvp failed, errno = 2 (No such file or directory)

Hello, I'm facing an issue of using gm on my OS X Capitan. I already install GraphicsMagick Below is my testing code. I'm trying to generate thumbnail for pdf document

var gm  = require('gm');

// Create JPG from page 0 of the PDF
gm("Travel.pdf[0]") // The name of your pdf
    .setFormat("jpg")
    .resize(200) // Resize to fixed 200px width, maintaining aspect ratio
    .quality(75) // Quality from 0 to 100
    .write("/tmp/cover.jpg", function(error){
        // Callback function executed when finished
        if (!error) {
            console.log("Finished saving JPG");
        } else {
            console.log("There was an error!", error);
        }
    });

Below is the output

There was an error! { [Error: Command failed: execvp failed, errno = 2 (No such file or directory) gm convert: "gs" "-q" "-dBATCH" "-dSAFER" "-dMaxBitmap=50000000" "-dNOPAUSE" "-sDEVICE=ppmraw" "-dTextAlphaBits=4" "-dGraphicsAlphaBits=4" "-r72x72" "-dFirstPage=1" "-dLastPage=1" "-sOutputFile=/var/folders/td/_f99nr0n0mz7x_k5g9j4zxnh0000gn/T/gmMft4YN" "--" "/var/folders/td/_f99nr0n0mz7x_k5g9j4zxnh0000gn/T/gmjkzXpN" "-c" "quit". gm convert: Postscript delegate failed (Travel.pdf). ] code: 1, signal: null }

Anyone has passed the issue please help. Thanks alot.