Sorry for the delay, I was very busy these last days.
If you want to add a gallery (from the Gallery extension) in a post is pretty easy.
You must first create the Gallery in the Gallery extension.
And then you write your blog post. You will find in the editor, an icon button for the gallery. Click on it, choose the gallery and a line will be added in the editor.
See the attached image.
I hope this will help you.