I just racked my brain for an hour trying to figure out why 3 registrants with Registration Error refused to be sync over to GTW even after I checked the Launchpoint integration and tried to manually set their program status to Registered. GTW doesn't like weird symbols in fields. One guy had an extra ' in his last name, the other two had | in the job title. Check for those if your webinar registrants in Marketo refuse to sync over to GTW.
Thanks, Venus! We ran into the same issue (a | symbol in the company name) on a separate GTW-MKTO sync error instance, as well! I searched through the Community and found that you can uncheck 'enable additional fields (company name, job title)' within the Launchpoint GTW integration. This way, symbols won't hold up the GTW-MKTO sync.