Why is なさい used in ごめんなさい when it is usually used as a command form?
2 Answers
免 is a somewhat old word meaning 'forgiveness'. There is a polite command form that goes お・ご~なさい, such as in おいでなさい or ご覧なさい. If you apply this to 免, you get ご免なさい, literally 'please forgive me'. This command form is also seen in おやすみなさい and おかえりなさい.

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Nothing at all's answer is fine the basics, but it could be misleading depending on how much Japanese you are planning to learn.
ご/おV-stemなさい is a polite command conjugation of the verb なさる.
なさる is the keigo equivalent of する (meaning it is respectful). Conversely, いたす is the self-humbling form.
So by using なさる you express respect for the person you are talking to and by using ご/おV-stemください you express it politely.
免ずる means to forgive and this is then conjugated to mean "please forgive me" by using a polite and respectful command conjugation.

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The following answer confirms it, なさる is the keigo verson of する, なさい is the imperative version of なさる, ご is a polite prefix, so ご免なさい is the polite imperative version of 免する: 免する -> (keigo) 免なさる -> (imperative) 免なさい -> (polite) ご免なさい. But what does ください have to do with it? – yk7 May 24 '23 at 04:21
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Actually there may be be an alternative "route": 免する -> (imperative) 免しろ -> (keigo) 免なさい... – yk7 May 24 '23 at 04:55