So I have what I was looking for, clear, written policy:
From CANFORGEN 201/18:
“THE GSM, OR BAR TO THE GSM, MAY ONLY BE AWARDED TO AN ELIGIBLE PERSON WHO IS IN AN ELIGIBLE LOCATION 180 DAYS OR MORE EITHER BEFORE OR AFTER THE DAY THEY QUALIFIED FOR THE GCS OR THEIR LATEST BAR TO THE GCS”
So expanding on this into examples:
A mbr who does 180 days in Kuwait, then gets tasked to Iraq for 30 days is eligible both.
A mbr who does 210 days in Kuwait gets the GSM with bar.
A mbr who does 210 days in Kuwait, then gets tasked to Iraq for 30 days is eligible for both and does not get a bar for the GSM.
A mbr who does 30 days in Iraq then at least 180 days in Kuwait gets both.
A mbr who does 30 days in Iraq then at least 210 days in Kuwait gets both, with a bar for the GSM.
Any variation which includes the GCS, 30 days in Kuwait, and being in Kuwait for the 180th day after qualifying for the GCS gets both. For example, some of my troops who've bounced around Lebanon, Jordan, and Bahrain on support tasks have qualified for medals related to operations there. They could also qualify for the GSM while all of this is going on.
If a mbr is returning to theatre for a TAV, the relevant date which decides whether they get a GSM or not is the 180th day after they qualified for the GCS, not the date that they returned from theatre.
Why this is relevant and not nitpicking is that I now have people who are in theatre for another eight months who are likely run into these qualifying criteria over the course of their time in theatre. I will not be here for all of that time, but I want them and their future supervisors to be aware of exactly what the criteria are. Not to write their schedules around collecting medals, but to get them recognition where appropriate.
If anybody is confused by any of this, please pipe up. I appreciate folks like PPCLI guy following up on this.