What is the ruling on money earned from working in a mixed-gender environment, and what is the ruling on possessions bought with this money, and how should one dispose of them after repentance?

We praise Allah for your repentance. Your previous work in mixed-gender environments and manufacturing items used for unlawful purposes was forbidden, as it constituted aiding in wrongdoing.

Sewing revealing women's clothing for those who wear them in disobedience is forbidden, as it involves aiding in sin. Anything that aids in disobedience is impermissible to sell or sew.

Repentance from ill-gotten wealth involves disposing of it by spending it on Muslim welfare and various acts of charity, such as giving alms to the poor and needy.

As for the money you earned from working in the factory—including the sewing machine—you are not required to give it away, unless the factory's work itself was forbidden. If you are in dire need, there is no harm in keeping what you require.