You really should not do what you are thinking of. Anti-phishing / link scanner software are clicking on all the links in your emails to check they are not dangerous URLs. If you unsubscribe people on a email link click, every of you customer equipped with a recent version of security software will be automatically unsubscribed.
Now, if you really want to go that route, the difficulty is that Marketo does not generate a "click link" event on this specific link. So you can either remove the specific codes in the link or use a "visits web page" trigger.
Also, with GDPR, you want to build out a subs mgmt center and not rely on clicks at all.