While there are numerous unresolved issues with WhoID/WhatID resolution (e.g., This
and
there is a puzzling omission on the reporting capabilities on GS Attachments object.
The Record ID field appears to function like a Context ID - so depending on the SOURCE of the Attachment (e.g., did you upload via Timeline, C/R360) you will get the ID for that record in that object.
If you Resolve WhatID in this report, you get two options:

First thing: There’s already Company and Relationship lookups directly on the object so it’s unclear what this solves here but it’s day 1 so maybe I’ll find out later

Secondly: Timeline is missing. Until this feature rolls out, 100% of your attachments are coming from Timeline (I guess either timeline or Drafts). As a result, resolving to either company or relationship is going to be useless because all the lookups will be blank until the C/R360 sections get used.
Please add timeline to the WHOID/WHATID resolution in this object.