Function to check if Gutenberg is active.

is_gutenberg_active.php

<?php
/**
 * Check if Block Editor is active.
 * Must only be used after plugins_loaded action is fired.
 *
 * @return bool
 */
function is_active() {
	// Gutenberg plugin is installed and activated.
	$gutenberg = ! ( false === has_filter( 'replace_editor', 'gutenberg_init' ) );

	// Block editor since 5.0.
	$block_editor = version_compare( $GLOBALS['wp_version'], '5.0-beta', '>' );

	if ( ! $gutenberg && ! $block_editor ) {
		return false;
	}

	if ( is_classic_editor_plugin_active() ) {
		$editor_option       = get_option( 'classic-editor-replace' );
		$block_editor_active = array( 'no-replace', 'block' );

		return in_array( $editor_option, $block_editor_active, true );
	}

	return true;
}

/**
 * Check if Classic Editor plugin is active.
 *
 * @return bool
 */
function is_classic_editor_plugin_active() {
    if ( ! function_exists( 'is_plugin_active' ) ) {
        include_once ABSPATH . 'wp-admin/includes/plugin.php';
    }
 
    if ( is_plugin_active( 'classic-editor/classic-editor.php' ) ) {
        return true;
    }
 
    return false;
}