How do I adjust the vertical gap between columns?

My site regularly needs to display a grid of small images with a caption below them and a title above them. The best way to do this that I’ve found so far is with a pair of column: the top row of columns has the title text, and the bottom row has the images and captions.

However, the total vertical spacing above and below all of these columns is huge, with no option to adjust it in the block’s settings. I’ve tried various custom CSS solutions found online, but none seem to work for me. I’ve also searched plugins, but did not easily find one that met my needs.

How do I reduce the vertical space between a column and the next/previous block?

The page I need help with: [log in to see the link]