Yeah the 'who cares' attitude would have annoyed me no end so I get where you are coming from. I also have a MIL who tells me how I should raise my baby and talks over the top of me if I ever try to explain why I make some of the choices I make. She has also whinged on a few occasions to my mother about my life decisions. I apparently was the one to blame for her son moving 20mins away from her and I make "weird" parenting choices, ohh and I have too many pets apparently. The fact that she tries to discuss these things with MY mother (possibly in the hopes that my mum will succeed in talking me around to her pov) really gets to me but she is so damn stubborn there really is no talking to her at all. I deal with her by using the old smile and nod trick for the day to day stuff and then when I say something she knows I mean it. Sometimes I slip and retort in a really happy cheerful voice like I am talking to my baby. I have found the bigger the smile you plaster on, the more people don't bother replying.
I would tell no one the gender except for your grandma if you can trust her to keep it a secret.