Add custom merge tags to Gravity Forms

gf-custom-merge-tags.php

<?php
add_filter( 'gform_replace_merge_tags', 'typewheel_custom_merge_tags', 10, 7 );

function typewheel_custom_merge_tags( $text, $form, $entry, $url_encode, $esc_html, $nl2br, $format ) {

	$custom_merge_tags = array(
		'{date_ymd}'  => date( 'Y.m-M.d', strtotime( $entry['date_created'] ) ),
		'{timestamp}' => time(),
		'{site_url}'  => get_site_url(),
		'{site_name}' => get_bloginfo( 'name' )
	);
    
    return str_replace( array_keys( $custom_merge_tags ), array_values( $custom_merge_tags ), $text);

}