While you try to log into your Gmail Account, on filling up the EmailID/Phone field with text and simultaneously click on the Next button, the text field for Password takes a delta amount of time to be clickable/interactable within the Viewport. Hence apart from just locating the xpath for Password field you have to induce some Explicit Wait i.e. WebDriverWait as follows:
WebDriverWait wait = new WebDriverWait(driver, 20);
WebElement password = wait.until(ExpectedConditions.elementToBeClickable(By.xpath("//input[@name='password']")));
password.sendKeys("your_password");