It is easy to create a gallery of photos.
The first thing is to add the photos you want to use to your Media Gallery.
This article explains how to add images to your Media Library.
Once the photos are in your Media Library (and remember that you can edit a gallery at any point, for example by adding more photos), click Galleries on your dashboard and then Add New Gallery.
Give your gallery a name.
Click Add to gallery to choose the photos you want to include. This takes you to your Media Library.
Hold down your SHIFT key to select more than one photo.
Click Select which will become active once you have selected at least one photo.
The photos will be added to your page.
Click on Set featured image in the sidebar on the right-hand side of the page. Choose a photo to set as the thumbnail for the gallery.
Click Publish. The gallery will automatically be added to the Photo Gallery page of your website.