I would say both of the previous replies are correct; it depends on the context. If you're in the middle of a conversation about kids, then "do you have a girl" would be asking if your child is a daughter or a son. If you're in the middle of a conversation about girlfriends, then "do you have a girl" would be asking if you have a girlfriend.

All three answers are correct but Wei's answer is the most complete. Also, using "do you have a girl" to mean "do you have a girlfriend" is common in the US but much less common in the UK.

差别大了，一个是问你有没有女儿，一个是问你有没有女朋友

If you are speaking about dating someone, asking "do you have a girl" is the same thing as asking "do you have a girlfriend".