Create a custom Gravity Forms merge tag

gforms-merge-tag.php

<?php
namespace HWID\SampleCode;

class GravityForms {
	public static function init() {
		add_filter( 'gform_custom_merge_tags', [ 'HWID\SampleCode\GravityForms', 'custom_merge_tags' ], 10, 4 );
		add_filter( 'gform_replace_merge_tags', [ 'HWID\SampleCode\GravityForms', 'replace_merge_tags' ], 10, 3 );
	}

	/**
	 * Add custom merge tag to the merge tags drop down in the form editor
	 *
	 * @param array $merge_tags
	 * @param int   $form_id
	 * @param array $fields
	 * @param int   $element_id
	 *
	 * @return array
	 */
	static function custom_merge_tags( $merge_tags, $form_id, $fields, $element_id ) {

		$merge_tags[] = array( 'label' => 'Name of Merge Tag', 'tag' => '{custom_merge_tag}' );

		return $merge_tags;
	}

	/**
	 * Replace custom merge tags in field content if they are found
	 *
	 * @param string $text
	 * @param $form
	 * @param $entry
	 *
	 * @return string
	 */
	static function replace_merge_tags( $text, $form, $entry ) {

//     Check to see if notification has the custom merge tag in it and replace it with the content of the tag
		if ( strpos( $text, '{custom_merge_tag}' ) !== false ) {

				$text = str_replace( '{custom_merge_tag}', 'Here it is!', $text );

		}

		return $text;
	}
}

GravityForms::init();