Gender euphoria is the opposite of gender dysphoria.

Like, if you woke up and were physically the opposite gender but mentally still felt to your core that you were your current gender. That discordance and, to some, disgust and physical pain, is gender dysphoria. Hard to imagine, sure.

Gender euphoria is the state that people experience in opposition to that. When what you see in the mirror matches up well with how you feel inside. Most people will take this state for granted, but for many trans people it is the exception, not the rule.

Hope this helps understand why euphoria is by no means an exaggeration in this sense :)