We have the OSS personality installed on our Tandem and I am able to download files from the Tandem Guardian space.

When I compare a Tandem directory like $SYSTEM.LEON.* to $DATA.LEON.* on my \OMHA node, I enter
sftp://auth.leon@10.11.23.30//G/system/leon in the left path and
sftp://auth.leon@10.11.23.30//G/data/leon in the right path

Via the OSS personality I can get to Guardian files, but there are limitations.

Guardian files have different structures, like Entry sequenced/relative/keyed sequence/unstructured/SQLmp/SQLmx. I primarily use Beyond compare to compare TEXT files and unstructured files so it works for me most of the time.