Some Wordpress plugins haven't yet been updated for the new Gutenberg editor, but you can still use them in your posts. Adding the Classic block will simulate the old editor for that piece of your blog post.
Once plugin we see this with is the Galleries by Angie Makes plugin. It will allow you to get a bit more free in creating your galleries for your blog posts. However, it does require using a Classic block to create the gallery. Here is how you can add that block to create the gallery you need.
First, you'll edit the post you're wanting to add the gallery to in Wordpress. Once there, select the 'add new block' button and choose Classic:
(If the Classic block doesn't show up right away, you can search 'classic' and it will pop right up)
After adding that, you should see your new Classic block. From there, you can select Add Media 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.