Are you sure? Because I think that a number of visual novels have been based on lesbian romances since the 90's. And no, I don't think that they are all porn.
Admittedly, it is arguable whether some visual novels are games, but the same can be said for the "Walking Sim" style.
No, I'm not 100% positive. I could be wrong about Gone Home being the absolute first game to center a lesbian narrative. It was the first to my knowledge though. I guess I should've said that it was the first I ever heard of at least and the first to achieve anywhere near its level of distribution and cultural notice. It's possible there were earlier, far more obscure examples. Some of them might have even been translated into English. Gone Home was the earliest such title that I'm aware of though.
It's not the best example either. I like the 2017 visual novel Butterfly Soup distinctly better. That's actually my absolute favorite game. It's also the only other game I'm aware of to center lesbian characters. It's a freebie that was created by one woman.