always set 15px margin bottom

assets/scss/snap-gallery.scss

@@ -6,7 +6,7 @@ $gap: var(--gap);
   justify-content: center;
   align-items: center;
   position: relative;
-  margin-bottom: $gap;
+  margin-bottom: 15px;
 
   /* Create equal columns in flexbox */
   .snap-image {