How can I add a custom attribute to Gutenberg core blocks?

You can add a custom attribute to any registered block by using the blocks.registerBlockType filter.

/**
 * Filter the attributes for all blocks to add custom ones
 *
 * Name can be used to only add the new attribute to a certain block.
 *
 * @param settings
 * @param name
 * @returns {*}
 */
function addCustomAttributes( settings, name ) {
    if ( settings.attributes ) {

        settings.attributes.customAttribute = {
            type: 'string',
            default: ''
        };

        settings.attributes.anotherCustomAttribute = {
            type: 'boolean',
            default: false
        };
    }
    return settings;
}

addFilter( 'blocks.registerBlockType', 'namespace/filterBlockAttributes', addCustomAttributes );