ShareGate Desktop does not support the migration of custom features, third party web parts, and third party apps. This is due to the number and the variety of third-party or custom solutions available and the disparity in standards.
ShareGate Desktop can migrate custom features and third party web parts or apps properly when these web parts are build in a similar fashion to SharePoint web parts.
Please ensure that your third-party or custom solution is installed and deployed at your destination and that all its related features are enabled before starting the migration.
A test run using the ShareGate Desktop trial version is strongly recommended in order to verify how the web part or app migration will be handled.