I think I figured it out. Again, super rusty w/ CSS, but here's what I did. Hope this helps!
/* Make sure to target your gallery. Each gallery has its own unique collection ID, which can be found via View Source. */
#collection-5f047ea7c17ab563ed1595c4{
.gallery-grid-item {
position: relative;
display: inline-block;
}
.gallery-caption-wrapper {
display: table;
height: 100%;
}
/* Caption Container */
figcaption.gallery-caption {
padding: 0;
position: absolute;
z-index: 999;
margin: 0 auto;
left: 0;
right: 0;
top: 0%;
width: 100%;
height: 100%;
}
.gallery-caption-content {
display: table-cell;
vertical-align: middle;
}
}