Ok, so I figured out a fix. In the file moode/lib/gradelib.php, in the function named grade_update, down near the end of the function is the code:

if (empty($g['userid']) or ($k != $g['userid'])) {
 debugging('Incorrect grade array index, must be user id! Grade
 ignored.');

 if (!empty($g['userid']) and ($k != $g['userid'])) {
 debugging('Reindexing!');
 if (array_key_exists($g['userid'], $grades)) {
 debugging('Entry in $grades exists for uid ' . $g['userid']);
 }
 else {
 $grades[$g['userid']] = $g;
 }
 }

Once you've done that, then go to the gradebook and select Categories and items from the drop-down menu:


Then, click the Synchronize legacy grades button:

I've written this code to be added to the workshop module so that the gradebook won't need to be synchronized in order to show its grades. The modification is for moodle 1.9.1 but I've reviewd the version changes file and it seems it haven't changed since 1.9.

I attach the lib.php to be overwritten (v 1.9.1). The modification is to be applied to mod/workshop/lib.php file.

2 new functions:

function workshop_grade_item_update($workshop, $grades=NULL){
global $CFG;
if (!function_exists('grade_update')) {
require_once($CFG->libdir.'/gradelib.php');
}

if (array_key_exists('coursemodule', $workshop)) {
$cmidnumber = get_field("course_modules", "idnumber", "id", $workshop->coursemodule);
$params = array('itemname'=>$workshop->name, 'needsupdate'=>1, 'idnumber'=>$cmidnumber);
} else {
$params = array('itemname'=>$workshop->name, 'needsupdate'=>1);
}

if ($workshop->visible == 0) {
$params['hidden'] = 1;
} else {
$releasetime = make_timestamp($workshop->releaseyear, $workshop->releasemonth, $workshop->releaseday, $workshop->releasehour, $workshop->releaseminute);
if($releasetime > time()){
$params['hidden'] = $releasetime;
}
}

if ($workshop->gradingstrategy == 0 OR ($workshop->grade == 0 AND $workshop->gradinggrade == 0)) {
$params['gradetype'] = GRADE_TYPE_NONE;
} else {
$params['gradetype'] = GRADE_TYPE_VALUE;
$params['grademax'] = $workshop->grade + $workshop->gradinggrade;
$params['grademin'] = 0;
}

if ($grades === 'reset') {
$params['reset'] = true;
$grades = NULL;
}

return grade_update('mod/workshop', $workshop->course, 'mod', 'workshop', $workshop->id, 0, $grades, $params);
}

function workshop_grade_item_delete($workshop) {
global $CFG;
require_once($CFG->libdir.'/gradelib.php');

return grade_update('mod/workshop', $workshop->course, 'mod', 'workshop', $workshop->id, 0, NULL, array('deleted'=>1));
}

And a modification in add/update/delete module instance functions:

In function workshop_add_instance (+/- line 140) you find

add_event($event);
}

After that, and before the return it should be added:

$workshop = stripslashes_recursive($workshop);
$workshop->id = $returnid;
workshop_grade_item_update($workshop);


In function workshop_update_instance (+/- line 917) where you find

if (time() > $workshop->assessmentstart) {
// regrade all the submissions...
set_field("workshop_submissions", "nassessments", 0, "workshopid", $workshop->id);
workshop_grade_assessments($workshop);
}


This should be added before the return

$workshop = stripslashes_recursive($workshop);
workshop_grade_item_update($workshop);


In function workshop_delete_instance (+/- line 512) you find

if (! delete_records('event', 'modulename', 'workshop', 'instance', $workshop->id)) {
$result = false;
}

The following should be added:

workshop_grade_item_delete($workshop);