It wont cost you anything, neither does it have any impact on your application. So you may go ahead and apply online.
GCMS really helps when you don't hear back from CIC, while the rest of the your respective month's group have received MR requests or worse - even the applicants from later months are receiving MRs and you havent. That is the time you should definitely apply for one. It may tell you about possible issues, it may not. All depends if the VO has left any sensible notes.
But it never hurts to apply for one.
https://atip-aiprp.apps.gc.ca/atip/client.do