
It might be as simple as “this is my gf ___” and letting them ask the questions on their own time. Sometimes I’ve found that overwhelming people with information can result in them only picking up part of it, causing misunderstanding down the line. Then again, that’s just my family and maybe yours are different?
Genuine advice: I came out as trans to my grandmother when she was, idk, 81 years old? She asked “what does that mean” once and then all her follow up questions were essentially “what do you expect me to do with that information.” Not in a shitty way! But in a very practical, “why have you told me this? What do you need me to do?”
Your grandparents, unfortunately, will not ever be able to FULLY understand this. If they do get it, it’ll take time, more time than a single conversation. So for the actual coming out, first give them the basic run down—my girlfriend is a trans woman, I am also a woman, I am a lesbian and this is the woman I am dating. Then be prepared for their follow up questions to be less “what even is gender” and more “what do we tell our friends when they ask if you’re single? Do we say you’re gay?”
This is, again, just my experience. But that repeated with a few of my older family members—my parents/aunts/uncles wanted to know why I was doing this, how I realized this. But my grandparents were all like “ok so am I still allowed to call you ‘sunshine’ or do I need to pick a new nickname because that’s too girly”
Just wanted to chime in that my grandparents were a similar age when I came out and live in deep red of deep red, and when my mom told them I’m trans, my grandma first asked wtf that was, and then when my mom explained my gma was like "oh" and then looked at my mom very accusatorily and said "Well I knew that. Do you think we're blind and stupid?" So 💀 you may get some... interesting reactions. She still struggles to not call us all "girls" but she’s good with name + pronouns.
Oddly enough my grandma is sort of the opposite lol. In that she always calls me her “grandson” (even when I’m not around, which is kind of sweet) but has a LOT of trouble with he/him. As in, I think she’s called me he like 3 times total. Luckily I never changed my name so the dead naming isn’t a problem.