Go to Setup > Developer Console > Debug > Open Execute Anonymous Window
Paste the following to the Apex Code Window, the click on the "Execute" button:
delete [select id from CRMG_GTM__Answer__c];
Once the above step is complete, remove what you previously pasted, then paste the following:
delete [select id from CRMG_GTM__Question__c];
Repeat, remove, copy paste "Execute" ... repeat for each of these items to paste.
delete [select id from CRMG_GTM__Poll__c];
delete [select id from CRMG_GTM__Survey__c];
delete [select id from CRMG_GTM__Webinars_Enrolled__c];
delete [select id from CRMG_GTM__Attendee__c];
delete [select id from CRMG_GTM__Session__c];
delete [select id from CRMG_GTM__Registrant__c];
delete [select id from CRMG_GTM__Webinar__c];
Once you've delete all of the Webinar details, go to GoToWebinar Users the Open the record you've created here ...
Click on "Get Webinars"
All of your data including the campaign details should arrive within 5 minutes or so.
Go to Campaigns and confirm
Go to "GoToWebinar User > Open your Account record > Click on "Get Webinars" to restart your sync