That’s exactly it, thanks Chris. With some extra help from Richard from the GF support team I’ve got it working.
add_action( 'gform_webhooks_post_request', 'gw_child_entries_to_repeater_event', 10, 4 );
function gw_child_entries_to_repeater_event( $response, $feed, $entry, $form ) {
$response_body_json = wp_remote_retrieve_body( $response );
$response_body_array = json_decode( $response_body_json, true );
$new_post_id = rgar( $response_body_array, 'id' );'
//Rest of function here
}