Incoming E-Mail settings is a SharePoint list feature that gives your users the ability to send E-mail messages and attachments directly to a list or library in SharePoint when activated.
You notice the Incoming E-Mail Settings are not preserved at the destination after your migration.
The ShareGate migration tool does not preserve the Incoming E-Mail Settings. The application cannot activate the settings in an on-premises farm and the feature is deprecated in Microsoft 365.
If you are migrating to a on-premises version of SharePoint, you can activate this setting manually after the migration.
The setting does not exist in Microsoft 365. You can get similar functionality with an Microsoft 365 Group.