includes/managers/class-fs-admin-notice-manager.php
function is_gutenberg_page() {
            if ( function_exists( 'is_gutenberg_page' ) &&
                 is_gutenberg_page()
            ) {
                // The Gutenberg plugin is on.
                return true;
            }

            $current_screen = get_current_screen();

            if ( method_exists( $current_screen, 'is_block_editor' ) &&
                 $current_screen->is_block_editor()
            ) {
                // Gutenberg page on 5+.
                return true;
            }

            return false;
        }
function is_gutenberg_page() {
            if ( function_exists( 'is_gutenberg_page' ) &&
                 is_gutenberg_page()
            ) {
                // The Gutenberg plugin is on.
                return true;
            }

            $current_screen = get_current_screen();

            if ( method_exists( $current_screen, 'is_block_editor' ) &&
                 $current_screen->is_block_editor()
            ) {
                // Gutenberg page on 5+.
                return true;
            }

            return false;
        }
function is_gutenberg_page ( ) { if ( function_exists ( 'is_gutenberg_page' ) && is_gutenberg_page ( ) ) { // The Gutenberg plugin is on.
 return true ; } $ current_screen = get_current_screen ( ) ; if ( method_exists ( $ current_screen , 'is_block_editor' ) && $ current_screen -> is_block_editor ( ) ) { // Gutenberg page on 5+.
 return true ; } return false ; }
Check if the current page is the Gutenberg block editor.

@author Vova Feldman (@svovaf)
@since  2.2.3

@return bool
Check if the current page is the Gutenberg block editor .