There was an error - compound class names not permitted, what should I do?

You are getting this error - compound class names not permitted because of the class name that you have used T-I J-J5-Ji ao0 T-I-atl T-I-JW ,

T-I
J-J5-Ji
ao0
T-I-atl
T-I-JW

are all different classes. When you use the following

      driver.findElement(By.className("T-I J-J5-Ji ao0 T-I-atl T-I-JW"));

the spaces between the class name confuses the method as to which class it should look for.

Try the following :
package testPkg;
import java.util.concurrent.TimeUnit;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.openqa.selenium.support.ui.ExpectedCondition;
import org.openqa.selenium.support.ui.ExpectedConditions;
import org.openqa.selenium.support.ui.WebDriverWait;

public class Gmail {
    public static void main(String[] args) {
        WebDriver driver = new FirefoxDriver();
        driver.manage().window().maximize();
        driver.manage().timeouts().implicitlyWait(30, TimeUnit.SECONDS);
        driver.get("https://www.google.lk/");
        driver.findElement(By.linkText("Gmail")).click();
        String s= driver.getTitle();
        System.out.println(s);
        driver.findElement(By.id("Email")).sendKeys("xyz@gmail.com");
        driver.findElement(By.id("next")).click();
        driver.findElement(By.id("Passwd")).sendKeys("password");
        driver.findElement(By.id("signIn")).click();
        String s1= driver.getTitle();
        System.out.println(s1);
        WebDriverWait wait= new WebDriverWait(driver,30);
        wait.until(ExpectedConditions.visibilityOfElementLocated(By.xpath("//div[contains(text(),'COMPOSE')]")));
        driver.findElement(By.xpath("//div[contains(text(),'COMPOSE')]")).click();
        driver.findElement(By.xpath("//div[contains(text(),'Send')]"));
        driver.quit();
        // TODO Auto-generated method stub
    }
}

Code tested on Ubuntu machine and working perfectly. The above code doesn't take into account adding the email address of the recipient and the message body.