Sharegate 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 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 trial version is strongly recommended in order to verify how the web part or app migration will be handled.