If you want to display your post’s images using Galelry+, you will need to follow these specific steps.
Switch to the Single Post Layout
Add a new Gallery+ block of the type you want to the Single page layout.
Configure it the way you want but set the Gallery and Embedding as follows:
Set Gallery to Content on the Images tab
Set block to embed on the Frames tab
Save the page.
You will notice the block will probably say "No images in selected gallery". That’s cool. It just means there’s none in the post you are viewing. On the live site, it will only show a gallery when one is available.
The following steps are for creating a gallery in the posts.
Add images to posts
But do not insert them into the post or insert the gallery
Add the shortcode to your post
Where this works really well, and why we use an embedded gallery, is this allows you to only show galleries on the posts you want to. If we had’ve left it as a standard block, it would show on every post, and often with that "No images" message.
The final product
And there we have our gallery in our post!