The Demographics report in Google analytics will show you an approximation of the gender and the age of your visitors. This data alone doesn’t tell you that much, but if you combine it with other data using segments and audiences you will be able to draw some pretty good generalisations about the group of people you should target with your ads.
In this video I explain some ways in which you can use demographics in Google Analytics together with segments.
The good thing is that most advertising platforms let you target users based on their demographics. With that in mind, understanding how to break down your different audiences based on such demographics might be useful.
The full article about demographics in Google Analytics can be found here.