The author of Gallery Factory has said this:
Unfortunately, “WP Offload S3” (I’ve checked the version from WP plugin directory, not Pro) is too focused on WP media, and it won’t be possible to use it with Gallery Factory. It’s possible in theory, but it will require some plug-in points to be provided by the author. On the other hand, “W3 Total Cache” seems to support GF, as it can monitor a custom list of folders and provide serving their content from S3. You could try free GF Lite version (look up “Gallery Factory Lite” in the WP plugin directory) and check how it works.
