Both “Молодец” and “Умница” can equally be said to men and women with no doubt. Here are samples: Она просто молодец - самостоятельно выучила русский! Он такой умница, что я в полном восторге! Ты умница! (no matter what gender is YOU) There is a masculine noun УМНИК too. But I reccomend non-native speakers avoid this word. Especially if you are not sure of the context. Умник often has a derisive meaning. In fact the peson called умник may not be smart at all. See a sample: Он перевёл весь текст совершенно неправильно! Тоже мне умник! And another: Умники из нашей Думы приняли очередной идиотский закон!

We use the word “Молодец” to show our praise, approval of a person possessing some positive qualities to do smth. perfectly, capably, deftly. “Молодец” is a masculine noun. For female we would use a noun “Умница” (Umnitsa)

It's mostly masculine. But it's possible to say this to women. You can also say "Ty umnitsa" (Ты умница), which is especially feminine.