How to detect the usage of Gutenberg

There are several variants:

• WordPress 4.9, Gutenberg plugin is not active
• WordPress 4.9, Gutenberg plugin is active
• WordPress 5.0, Block Editor by default
• WordPress 5.0, Classic Editor plugin is active
• WordPress 5.0, Classic Editor plugin is active, but in site console in “Settings > Writing” the option “Use the Block editor by default…” is selected

All the mentioned variants can be processed by the following code:

/**
 * 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;
}

Function returns true if block editor is active by any means, and false – in the case if classic editor is here. This function must only be used after plugins_loaded action is fired.

P.S. Due release of version 1.2 of Classic Editor plugin, code is updated, as classic-editor-replace options now takes values not replace and no-replace, but classic and block.