Function to check if Gutenberg is active.

is_gutenberg_active.php

	/**
	 * Check if Gutenberg is active.
	 * Must be used not earlier than plugins_loaded action fired.
	 *
	 * @return bool
	 */
	private function is_gutenberg_active() {
		$gutenberg    = false;
		$block_editor = false;

		if ( has_filter( 'replace_editor', 'gutenberg_init' ) ) {
			// Gutenberg is installed and activated.
			$gutenberg = true;
		}

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

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

		include_once ABSPATH . 'wp-admin/includes/plugin.php';

		if ( ! is_plugin_active( 'classic-editor/classic-editor.php' ) ) {
			return true;
		}

		$use_block_editor = ( get_option( 'classic-editor-replace' ) === 'no-replace' );

		return $use_block_editor;
	}

Nice snippet! Thanks for sharing :)