When you add a new block style, you can also make it the default style in the editor.
Revisiting our gallery block where we hid the captions, we only need to add one new line: isDefault: true.
Our example codes would then look like this:
JavaScript:
wp.blocks.registerBlockStyle('core/gallery', {
name: 'slug-hide-caption',
label: wp.i18n.__('Hide captions', 'text-domain'),
isDefault: true
});
Or, if you added the block style with PHP:
register_block_style(
'core/gallery',
array(
'name' => 'slug-hide-caption',
'label' => __( 'Hide caption', 'text-domain' ),
'inline_style' => '.is-style-slug-hide-caption figcaption { display: none; }',
'isdefault' => true,
)
);