Symptoms
There is no value in the Site Owners field for a given site.
Details
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 ShareGate Migrate, it most likely means that the group is not set correctly.
Solution
To set up the Owners group of your site, along with the Members group and the Visitors group:
In your browser, navigate to your site.
In the address bar, add the following to the site URL:
<MySiteUrl>/_layouts/permsetup.aspx
This page must be accessed using the site URL as there is no way to navigate to it through the SharePoint navigation.
If the Create a new group radio button is selected, no groups are set on this page.
Select Use an existing group and choose the appropriate group for each category.
Click Ok.
Refresh your site in ShareGate Migrate.
You should see the appropriate users in the Site Owners field.