placeholder-gravityforms.php

<?php

add_filter('gform_field_content', 'add_placeholder_attr', 10, 3);

/**
 * Add a placeholder attribute to text inputs
 * 
 * @param string $content
 * @param array $field
 * @param mixed $value
 *
 * @return string
 */
function add_placeholder_attr($content, $field, $value)
{
    if ($field['type'] == 'text' || $field['type'] == 'email') {
        $label = $field['label'];
        $placeholder = "placeholder='{$label}' value='{$value}' ";
        $content = str_replace("value='' ", $placeholder, $content);
    }

    return $content;
}