"G'mar hatimah tova" is the traditional Yom Kippur greeting. It means "May you be sealed for a good year in the book of life" in Hebrew. "Tzom kal" is also a fitting Yom Kippur greeting, as it means "easy fast" in Hebrew.
Yom Kippur is a day of fasting for Jewish people, and the holiest day in the Jewish calendar. Jewish people believe that God seals the fate of humans for the upcoming year, based on their actions, on Yom Kippur. The 10 days between Rosh Hashana and Yom Kippur are still considered the new year. So it is appropriate to wish a person "L'Shana Tova" or "happy new year" on Yom Kippur as well.