Sharegate displays the following error:
This document cannot be downloaded from Google Drive because it is linked to a third-party application. Please consult the third-party's documentation for instructions on how to export the document.
Google Drive allows you to create documents that are associated to a third-party application. Since these documents are directly associated to the third-party service with Google Drive only being used as a raw data store for the contents of the documents, Google Drive does not allow you to download the contents of the document. You can see this behavior for yourself by right-clicking on the document in the Google Drive interface. The "Download" option should not be shown while it is normally available on other documents. As such, Sharegate cannot download the contents of the document either.
Since Google Drive does not permit the download of the document contents, the only solution is to consult the third-party service's site or documentation and determine if they offer a way to export the document. Unfortunately, this behavior cannot be automated by Sharegate at this time. For any additional details, please contact the third-party service's support. They may offer an alternative way of exporting the documents that you can use.