Cacher is the code snippet organizer for pro developers

<?php
/**
 * Gravity Wiz // Gravity Forms // Modify Date Format for Custom Field
 * http://graivtywiz.com/
 */
add_filter( 'gform_post_data', function( $data ) {
	// Update both instances of "my_date_custom_field" to your custom field name.
	// Update 'Y-m-d' to your desired date format.
	$data['post_custom_fields']['my_date_custom_field'] = date( 'Y-m-d', strtotime( $data['post_custom_fields']['my_date_custom_field'] ) );
	return $data;
} );