Currently, Cyberduck is incompatible with SFTP Gateway. When using Cyberduck, you're unable to download files over 10MB in size.
Cause of the Issue
The reason for the inability to download files over 10MB is due to Cyberduck using Random File Access to download files in concurrent chunks. This is an issue as SFTP Gateway does not support Random File Access.
Cyberduck's SFTP behavior is always using concurrent chunk download, regardless of settings, and because of this it will be unable to download files any larger than 10MB.
We may support Random File Access in the future, although this isn't certain, as it can be difficult to implement with Object Based Storage.