There's alot of information, much of it isn't too exciting. For reference I'll use my notes, yours might have slightly different page #'s but, at last it's a reference point.
Page 8 Eligibility
Here you should see the name of the PA, a Status (not started, in progress, etc), and the last time this status was updated.
Admissibilities
Here you'll find security, criminality, and medical checks, same as above showing their status and update times.
then scrolling waaay down you'll find
History (mine starts page 20)
This area shows (newest first) updates to your file. You can see when/where it's travelled, when checks are being done, etc.
then again, waay done on page 34 you'll find
Notes
This might be the most interesting part, you can see what the officer has to say about your file.
See Eagle333's thread for more details: http://www.canadavisa.com/canada-immigration-discussion-board/gcms-notes-well-explained-has-never-before-until-now-t390292.0.html
