You need the "Classic" block. When adding a new block, search for classic and insert that block. It's under the Formatting section. You'll get the same old toolbar, at the end of which you'll find the purple Shortcode Editor button, as usual. Remember, the shortcode is only needed when you want to use special features and sources. Starting from JIG v3.9.2, JIG can automatically take over the regular Gutenberg gallery block (that's just a WordPress gallery).