"He is gentle to our pets."
I had the same problem. The joke is that right before I had to translate this into French, I had to translate the exact same sentence (il est gentil...) into English and Duolingo suggested "gentle towards" too.
Why is it "Il est doux" in this question, but when it asks you to translate "she is gentle...." it marks this incorrect and specifies "elle est douce". I'm a bit sick of getting these wrong.
Far from an expert, but I think it is that doux is masculine and douce is feminine.