First, select the add image button in the editor. You will then go straight to the media manager.
In the media manager you can select multiple images.
Then click om to go to the next page.
You can choose which layout you want for your gallery. The layout will only differ when it is displayed on desktop as for mobile a gallery will always be 2 rows.
Add the gallery to your article by clicking Add image(s). You will now see the gallery in the layout you have chosen in the article. See below for example desktop if you have chosen 3 in a row.