Hello and welcome! You are certainly in a difficult situation.
As a lifelong Heinlein fan (the author of Stranger in a Strange Land) my ideas about poly and relationships have been shaped by concepts in his books since an early age. However, not everyone, it seems, is predisposed to be able to practice the "ideal" represented.
Mixing psychiatric problems into the mix only adds to the strain. Just based on what I have read in what you posted...it sounds like C and her boyfriend may not be the best match for you and G in your current situation. Perhaps if you took sex off the table G would be more comfortable with you providing C with the love and caring that you want to give her. Just a thought...could you be C's "platonic boyfriend" for some period of time - free to cultivate emotional intimacy, spend a significant amount of time with her, etc - with an idea to revisit this boundary in 2-4-6 months?
I think I understand that C's diagnosis - the fact that it is progressive and you feel you have a limited time to experience your old friend as the person you knew her as - lends a sense of urgency to the issue. But focusing on the quality of your time together rather than the quantity may be useful. (Perhaps you could even negotiate with G a limited number of sexual encounters spaced over a reasonable time-frame that would be acceptable even if uncomfortable?)
Another consideration in how you proceed: How much are you willing to risk losing/disappointing G (with whom you have the potential / intention to have long-term plans) to realize your unrequited feelings for C (with whom you only envision shorter term possibilities given her diagnosis)?
Just some thoughts based on one post...
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (22+ yrs)
Dude: hetero poly male, live-in boyfriend (3+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe