The sharing settings allow you to define restrictions on the way your users can share content in your environment.
Symptoms
Sharing settings are not preserved, the default setting of the tenant is used instead.
Details
ShareGate Migrate does not preserve the Sharing by domain list when the setting is set to Anyone.
The app avoids settings that are more permissive than the settings that are set at the tenant level.
ShareGate Migrate is unable to copy your sharing settings if your are not tenant admin.
ShareGate does not preserve your sharing settings when you merge into an existing site collection or migrate to a subsite with Copy structure and content.
Solution
Make sure you are Tenant admin or SharePoint admin if your migration supports the sharing settings.