What does it mean when you're the different gender in your dreams?

it is believed that if you are a girl and you dream you are a guy, you need protection and leadership in your life, or if you are a guy dreaming of being a girl, maybe you struggle with your masculenity

Another response:

Everyone has both genders in the subconscious mind. In men it is referred to as the anima, in women, the animus. This part of the mind holds the characteristics of the opposite gender: gentleness and submissive qualities in men and strength and aggression in women. It is perfectly normal for the anima/animus to be expressed in dreams.