I'm so sorry for how this hurts you. And I probably understand it better than anyone as I have wanted for Josh to want to get married for 10 years now. But I've taken stock of our situation & realize that I have a good man - just as you do and love the life we are living together. No bonds of marriage are going to improve that. In fact - Josh worries that marriage could wreck it.
I don't believe that Gabe is going anywhere. I hope you can appreciate what the 2 of you have and continue to grow as a couple, especially after Marc is gone.
Hugs to you as I know how much those words hurt. But ask yourself if you are happier with your current situation or if he was gone. Sometimes we have to accept the compromise and just move fwd.
