How can I shut down any religious homophobia coming from my Grandmother?

If she's willing to listen, you can try to inquire about her reasons for being homophobic and discuss it together. Maybe she never had a chance to question her point of view, and your arguments could at least make her consider it. If she won't listen to reason nor empathy, sometimes it's simply better to try to avoid the topic and only share this part of you with the people who accept it and deserve to know the real you.