Attendees are required to register before they attend a GoToWebinar session. While there is no way around the registration process, you can limit the registration questions to just their name and email address.
If your audience is already registered in some other system that you use, there are some integrations available that you may be able to use to import and register your audience: https://www.gotomeeting.com/integration-marketplace
We also have an API available here: https://goto-developer.logmeininc.com/gotowebinar-api-overview
It's not possible to have a generic link for all GoToWebinar attendees to use, but if this is critical to you, you could use GoToMeeting instead (as long as you do not need more than 250 attendees or the other GoToWebinar features).
There is an integration with Mailchimp via Zapier that you can use, please read this support article: https://support.logmeininc.com/gotowebinar/help/add-gotowebinar-registrants-to-mailchimp-g2w160007