you only need some math calculation to get free space in GB:

Code:
  private void button1_Click(object sender, EventArgs e)
        {
            ManagementObject disk = new ManagementObject("win32_logicaldisk.deviceid='e:'");
            disk.Get();

            double diskSize = (ulong)disk["Size"] / (Math.Pow(1024, 3));
            double diskFree = (ulong)disk["FreeSpace"] / (Math.Pow(1024, 3));
            double percentFree = (diskFree * 100) / diskSize;

            label1.Text = String.Format("Your Total Disk Size on Drive E is: {0:#.##} Giga Bytes", diskSize);
            label2.Text = String.Format("Your Free Disk Space on Drive E is: {0:#.##} %", percentFree);
          
        }