Is one sinful for looking at the face of an unveiled girl out of curiosity, and is this sin forgiven through Dhikr (remembrance of Allah)?

Looking at the face of a non-mahram (definition) woman with lust is forbidden by the consensus of jurists. As for looking at her without lust or fear of fitna, the preponderant opinion according to us is that it is forbidden.

This type of sin is expiated by effacing good deeds such as prayer and fasting, based on the Almighty's saying: "Indeed, good deeds do away with evil deeds."

Ibn Taymiyyah mentioned ten reasons that lift the punishment for sin, among them repentance, seeking forgiveness, effacing good deeds, expiating calamities, and the intercession of the Prophet (peace and blessings be upon him), among others.