Make the default comment_form more Gravity Form-like, with class names and wrappers.

functions.php

<?php

/**
 * _site_comment_form_top
 * @desc	Wrap the comment form fields (not submit) in a <div> (open)
 */
function _site_comment_form_top() {
	echo '<div class="gform_body">';
}
add_action('comment_form_top', '_site_comment_form_top');

/**
 * _site_comment_form_after_fields
 * @desc	Wrap the comment form fields in a <div> (close)
 */
function _site_comment_form_field_comment($field) {
	return $field . '<!-- end of div .gform_body --></div>';
}
add_filter('comment_form_field_comment', '_site_comment_form_field_comment');

single.php

<?php
$commenter = wp_get_current_commenter();

comment_form(array(
	'comment_notes_before'	=> '',
	'comment_notes_after'	=> '',
	'format'		=> 'html5',
	'label_submit'		=> 'Submit',
	'fields' => array(
		'author'	=>	'<div class="gfield gfield_contains_required field-author">' . "\r\n" .
						'<label for="author gfield_label">Name <abbr class="gfield_required" title="Required">*</abbr></label> ' . "\r\n" .
						'<div class="ginput_container">' . "\r\n" .
							'<input id="author" name="author" type="text" value="' . esc_attr($commenter['comment_author']) . '" size="30" aria-required="true" />' . "\r\n" .
						'</div>' . "\r\n" .
					'</div>',
		'email'		=>	'<div class="gfield gfield_contains_required field-email">' . "\r\n" .
							'<label for="email" gfield_label>Email <abbr class="gfield_required" title="Required">*</abbr></label> ' . "\r\n" .
							'<div class="ginput_container">' . "\r\n" .
								'<input id="email" name="email" type="email" value="' . esc_attr($commenter['comment_author_email']) . '" size="30" aria-required="true" />' . "\r\n" .
							'</div>' . "\r\n" .
						'</div>',
		'url'		=>	'<div class="gfield field-url">' .  "\r\n" .
						'<label for="url gfield_label">Website</label> ' . "\r\n" .
						'<div class="ginput_container">' . "\r\n" .
							'<input id="url" name="url" type="url" value="' . esc_attr($commenter['comment_author_url']) . '" size="30" />' . "\r\n" .
						'</div>' . "\r\n" .
					'</div>',
	),
	'comment_field' 	=>	'<div class="gfield gfield_contains_required field-comment">' . "\r\n" .
						'<label for="comment gfield_label">Comment <abbr class="gfield_required" title="Required">*</abbr></label> ' . "\r\n" .
						'<div class="ginput_container">' . "\r\n" .
							'<textarea id="comment" name="comment" cols="45" rows="8" aria-required="true" placeholder="Leave a comment"></textarea>' . "\r\n" .
						'</div>' . "\r\n" .
					'</div>',
)); ?>