In save_post hook, to check if the post is published via gutenberg, use this

add_action( 'block_editor_meta_box_hidden_fields', 'add_gutenberg_hidden_fields', 10, 1 );
function add_gutenberg_hidden_fields( $post ) {

	echo '<input type="hidden" name="your-field" value="some-value">';
	echo '<input type="hidden" name="is_gutenberg_post" value="1">';
}

then inside save_post callback, use

if ( isset( $_POST['is_gutenberg_post'] ) ) {
    // came from gutengerg
}