When migrating documents containing Quick Parts, the metadata in the document is unavailable or invalid at your destination.
Unfortunately, Quick Parts are not supported by Sharegate.
Quick Parts are a way to use your column metadata in your Office documents.
Metadata in Quick Parts are linked to SharePoint columns via their unique identifier. When migrating to a destination, the identifiers will be different. Sharegate never modifies the contents of documents, so it cannot convert these identifiers to allow Quick Parts to function at the destination.