How to find the gender of the user for a mobile build?

Hello,

TARGET : Mobile

MARKETPLACE: Amazon, Google Play & App Store.

I am using analytics and I want to find the age & gender of the user who bought my game. How to achieve it?
I do not want to ask it from the user. I also see that age might be a difficult task but is there a way to find the gender.

Thanks and appreciate your help.

Make them login using facebook and then im sure you could figure a way to extract gender