Hi Sophie,
You may try two things to add spacing below the gallery.
1. Insert the Gallery Block within a GB Container Block which would allow you to easily modify the spacing.
2. Or, add a CSS like this in Appearance > Customize > Additional CSS to modify the bottom margin of all WP Gallery Blocks:
.wp-block-gallery {
margin-bottom: 10px;
}
Alternative if you want padding bottom:
.wp-block-gallery {
padding-bottom: 10px;
}
Kindly modify 10px
to your preferred value.
Hope this helps!