When grade sync is turned off on an activity (e.g. a quiz) in the tool, the option to 'add to gradebook' is still offered in the platform's content selection window. This is confusing for users who would reasonably expect to be able to add it to their gradebook if offered. 

As identified by Jake here https://moodle.org/mod/forum/discuss.php?d=462358 this behaviour is based only on whether the activity supports grading at the moment. The code can be found here: https://github.com/moodle/moodle/blob/aea624fcb0229d40568850c9de808de2155d4beb/enrol/lti/classes/output/renderer.php#L88 

Ideally, the content selection window would reflect whether grade sync is turned on for that activity, removing the option to 'add to gradebook' when turned off.