I have found a way of finding gid and sid from the userid, I just post it here in case it is of any use:
$cm = get_coursemodule_from_instance('assign', $assignment->id, $courseid);
$submission = $DB->get_record('assign_submission', array('userid' => $userid,
'assignment' => $assignment->id));
$sid = $submission->id;
$grade = $DB->get_record('assign_grades', array('userid' => $userid,
'assignment' => $assignment->id));
$gid=$grade ->id;
$urlparams = array('id' => $cm->id,
'sid'=>$sid,
'gid'=>$gid,
'plugin'=>comments,
'action'=>viewpluginassignfeedback,
);
$assignurl = new moodle_url('/mod/assign/view.php', $urlparams);
R egards,Mari Cruz