I came out to my parents my senior year of high school which was (Im showing my age here but) 12 years ago. I knew I was gay when I was very young and like you, the thing that held me back was the homophobic comments. My mom would make comments whenever she would hear or see something about a gay person and say "Thats not the way God made you" I think your grandma (like my mom) truly did know that you were gay but didnt want to accept it and thought that by making those comments that you would "choose" not to be gay. I know its not a choice, but its hard for people who arent gay to understand how we know that we are gay.

Flash forward six years later when I had my first "real" boyfriend. I took him to meet my parents and they absolutely loved him and would ask about him whenever they would see or talk to me. You just have to give your mom and grandma some time to let this sink in and for them to come to terms with it. No, I do not agree with your mom telling your grandma because it was not her place to tell her. But the bottom line is, you just have to give them some time. Its not going to be overnight or next week. It took my mom a couple years and my dad a year longer to come around to the point where it doesnt even bother them in the slightest anymore. Dont start any fights, dont instigate any arguments, and dont bring it up unless they do. One day down the road when the time is right, you can bring it up and have a serious talk about it with your mom and grandma.