I investigated the states set using React Dev Tools and found the preview modes are set under the deviceType key. Went through the Gutenberg source and came across the __experimentalGetPreviewDeviceType function.
/**
* Returns the current deviceType.
*/
const { deviceType } = useSelect( select => {
const { __experimentalGetPreviewDeviceType } = select( 'core/edit-post' );
return {
deviceType: __experimentalGetPreviewDeviceType(),
}
}, [] );
I’m using this to conditionally set the CSS classes. Worked for me and I hope the Gallery block is doing the same way.