Adding grid pictures CSS

Hello, I'm trying to add a grid gallery of pictures to my site.  I can't get the pictures in a grid format when I add a custom css. Is there a way to add grids?  I attached a picture to showcase the issue, is there a way to fix this?  Thanks in advance

Writing display grid in CSS sadly isn't going to work, you need to set the gallery style to grid if that's what you want.

If you share your website URL and the correct page (with a live template) then it's typically easier to provide more specific help.