As long as you have "Use Compression" checked in your connection settings you should expect that files will land in GCS buckets with the "text/plain" content-type and gzip encoding. That said, when downloading the file in the GCS UI or via command line tools GCS will decompress the file to a .txt file before transfer.
If you're building a workflow to move data from GCS to other platforms ensure that you account for this.
To learn more, please visit our docs.