There is no evidence that people with pre-existing gynecomastia will be more susceptible to further development of tissue while taking finasteride. I myself have a small amount of tissue left over from puberty, but it has not gotten any worse in 1.5 years on finasteride.
However, if you are an adult and already have actively developing tissue, you should stay away from finasteride. Also, there are a number of factors that will significantly increase your risk of developing gyno, such as being overweight, having high estrogen levels, alcoholism, and even marijuana use has shown a link. Interestingly, when I started finasteride I was a bit overweight, drank like a fish and smoked marijuana quite regularly, but never developed gyno. I experienced some breast pain initially for a short time, but this went away.