Well, I think experiences are very variable but I do think GU exist. Might mean a VO is working on the file or reviewing it. I got my BIL on Sept 18 and because I kept re logging on my GC key, I saw that my Med had passed. Didn’t get message until Sept 19.
Then, I got a GU that application had been updated on GCkey 03/10, didn’t find anything but when I checked my tracker my eligibility had changed from “not started” to “completed”. But again, updates may be variable for everyone, especially as the IT system won’t be 100% efficient.