I asked the question, I found the answer: I managed to obtain my GCMS client notes.
I just filled out the typical ATIP online application and instead of providing an application/case number I just put my UCI and then filled out the comments section explaining I want my GCMS client notes, not case notes linked to a particular application.
The result came in a tabular form containing notes, date, agent.
It was interesting to observe, as others noted on the forum, that some notes resulting from calls to CIC call centre found their way there instead of case notes.