I know two ways that you can view a gigapan panorama on Google Earth:
1. On a page of gigapan.org:
To view a gigapan panorama when you are on a page of gigapan.org (e.g: Most Popular Gigapans, Most Recent Gigapans, or even a user’s profile), click on the “View in Google Earth 4.2+” link that appears below the panorama and its descriptions, as shown in the photo below.
2. When viewing a gigapan:
When you are viewing a gigapan panorama on its own page, you can easily view the gigapan on Google Earth by clicking on the “View in Google Earth 4.2+” link which is located between the snapshots bar and the Google Map box, on right side of the page.
If you see no link to view the gigapan on Google Earth, the gigapanner has not geo-located that gigapan on Google Earth.
You may also like to know: How to View All GigaPan Panoramas on Google Earth.