Spacing photos on gallery page not working

Question

I have the page working otherwise but the spacing isn't when added to the main.css page...

Here is a codepen of what I have working: https://codepen.io/anon/pen/rQxxyJ.enter code here

Here is the codepen that has my main.css file (disregard the HTML): https://codepen.io/AJ2021/pen/pQzmORenter code here

Any help is greatly appreciated!