Mr. G sounds like a special person in your life. I know you have lived with the tradition of being mono, and poly is new ground for you. I wouldn't be surprised if a younger you had grown up with a tradition if being straight before you accepted your gayness. I know I struggled for years with being poly before I self-identified as poly. The fact you're here seeking insight tells me you at least have the potential for poly love.
In some ways, your situation sounds pretty good. Mr. G sounds like and awesome sir who really likes his boy. Yes, there is this other person, but doesn't Mr. G radiate enough caring for you regardless? How cool is it that he might have the capacity to care for his other dear friend and yet be able to pour his passion out for you?
Oh, but you've said this or that and messed it up... Blah blah blah wah wah wah. Don't worry about that. If you lay your cards out (face up--no hole cards, good sir!) and embrace the potential you share, you may find a happy place again.
As a practical note, it sounds like you need reassurance and that he might not be good at that. (Neither if these is a bad thing, but biweekly we react can be bad.) I suggest a few simple things:
1) Tell him bluntly when you feel the need for reassurance and what you need. "Remind me my ass looks great." Or "I really like it when you put your hand on my shoulder when I'm near you."
2) When it comes to the snuggling while sleeping idea, consider a totem. If he can't be close it isn't inclined to be nearby, you might be comforted by a reminder. This could be as simple as a picture of him (or the two of you) that you can see from your pillow. Or you might ask him to but you a special pair of pajamas so you can sleep with his gift wrapped around you. Better yet, if your physiques work fit, you could ask for a shirt if his to sleep in so you have something if his against your skin as you dream.
Just remember, you are wonderful as you are. Mr. G sounds wonderful, too. Maybe you will find that your mono tradition is something you can set aside. (Mr. G's OSO sounds like a decent dude, too. Not bad.). Or perhaps you have confirmed you are mono. Just don't met growing pains scare you away.
All the best to you and yours. You know where we are if you want more (questionably mediocre