When migrating documents containing Quick Parts, the metadata in the document is unavailable or invalid at your destination.
Quick Parts are a way to use your column metadata in your Office documents.
Unfortunately, Quick Parts are not supported by Sharegate.
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.