There is no value in the Site Owners field for a given site.
Even if you can see the default MySite Owners group on your site, it is possible that it is not set as the Owners group of your site. If you don't see the group members in the Site Owners field of the ShareGate migration tool, it most likely means that the group is not set properly.
To set the Owners group of your site, along with the Members group and the Visitors group:
- In browser, navigate to your site.
- In the address bar, add the following to the site URL:
This page must be accessed using the site URL as there is no way to navigate to it through the SharePoint navigation.
On this page, if the Create a new group radio button is selected, it means that no groups are set.
- Select Use an existing group and choose the appropriate group for each category.
- Click Ok.
- Refresh your site in the ShareGate migration tool.
You should see the appropriate users in the Site Owners field.