No need for a third-party plugin! Here’s how to do it while composing a post.
Say, you’re done uploading the photos.
1. In the Editor, tap the + icon on the toolbar.
2. Go to the WordPress Media tab.
3. Select all the photos you want to include in the gallery.
4. Add them to your post! The app will automatically prompt you the same in the screenshot. There are some styles to choose from! Just tap on each to preview.
Note that you can’t preview the gallery in the editor.
You’ll probably just see a code added to your post, but the gallery will appear on a browser.