select multiple photos to add to a gallery with a option-click or command-click -- or even just a single click for each image needed -- should be standard.
many people are using multiple photos across different galleries. Choosing 10 images already uploaded in a new gallery means you have to search image ten times, and if the image you want has been uploaded earlier, you have to scroll down which takes forever.
Other sites enabled multiple selections years ago. Why hasn't squarespace?