Help finding selectors and classes

Hi,

I'm new to CSS and have a hard time finding the correct selectors and classes.

On this blogpost: https://www.a-c-d.net/colorful-space-cowboy-illustrations-by-dalek2020/

I have a wordpress theme and use gutenberg image gallery (wordpress).

I want to add a space between each image except the last one.

I gave the gallery Additional CSS Class: space_gallery

I tried different classes and selectors without luck.

Thanks

Ben