Masonry Galleries & Gutenberg
The new WordPress editor, codenamed Gutenberg, comes with a new gallery block that lets you easily pick the number of columns, and quickly put together a nice image collage.

If we utilize the ability to also add a custom CSS class, we can rearrange this gallery relatively easily, using just a little bit of additional CSS.
Start by opening the Block Settings panel, and adding custom-gallery
in the Additional CSS Class field:

We can now write a snippet of custom CSS to hook into that and tune the design. Here's a bit of CSS that'll turn it into a masonry layout.
.custom-gallery {
column-count: 2;
column-gap: 0;
padding: 0;
display: block;
}
@media (min-width: 782px) {
.custom-gallery {
column-count: 3;
}
}
@media (min-width: 1440px) {
.custom-gallery {
column-count: 4;
}
}
.custom-gallery .blocks-gallery-item {
display: block;
width: auto;
margin: 0;
}
.custom-gallery .blocks-gallery-item figure {
height: auto;
}
If you'd like an example of what this looks like, head to my Projects page.
No doubt someone will soon enough create a dedicated block just for masonry-style galleries. In the meantime, this trick might hold you over.