Yeah as Ieesha said, it depends on the situation and whether there's any reason you would want to keep it on the down low. For us we announced the gender with both our kids to my family and husband's family (parents and siblings) and we were happy for them to tell the gender if anyone asked (my family and husband's extended family lives abroad) or we would say if anyone asked.
I guess it depends on the situation, if I'm talking to people and it comes up in conversation I'll tell them we're having a girl. Do you want to keep it a surprise or are you excited to tell people now? :)