Adding the Classic Block will simulate the old editor for that piece of your blog post.
Add a Classic Block
First, you'll edit the post that you're wanting to add the classic block to in WordPress. Once there, select the Add Block button and choose Classic.
After adding that, you should see your new Classic block.
From there, can choose from the classic block options, including creating a classic gallery by by choosing the Add Media option to pull up your media library.
Now that you're in your media library, you can select the images you're wanting to add to the gallery.
From there, you can edit the different settings for your gallery, such as the image size, gallery type, number of columns, and more.