Hey Diljeet, following test script allows you to capture tooltip on gmail webpage using Selenium Webdriver:
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.openqa.selenium.interactions.Actions;
public class TooltipGmail {
  public static void main(String[] args) {
    WebDriver driver=new FirefoxDriver();
    driver.manage().window().maximize();
    driver.get("https://accounts.google.com/SignUp?service=mail&continue=https%3A%2F%2Fmail.google.com%2Fmail%2F<mpl=default");
    driver.findElement(By.xpath(".//*[@id='GmailAddress']")).click();
    WebElement username_tooltip=driver.findElement(By.xpath("html/body/div[2]/div[1]"));
    Actions builder=new Actions(driver);
    builder.moveToElement(username_tooltip).perform();
    String tooltip_msg=username_tooltip.getText();
    System.out.println("Tooltip/ Help message is "+tooltip_msg);
 }
}