Gender Euphoria is a psychological condition which consists of comfort or even joy when thinking about one's true Gender identity, often accompanied by a strong desire to change one's sex to better match their identity or to be called the correct gendered language.
Euphoria is actually the opposite to 'Dysphoria', or the absence of pleasure. It is thus pleasure, and is not directly attached to gender identity.
Top Euphoria is a term most often used to describe someone's comfort or even joy when thinking about their preferred configuration, such as transmasculine people feeling top euphoria thinking about lack of breasts and masculine shoulders and arms, or transfeminine feeling top euphoria thinking about the presence of breasts on more feminine build.
Bottom Euphoria is a term often used to describe one's comfort or even joy when thinking about their genitalia matching their gender identity.