Q. Do we greet one another with Gut Yom Tov on Yom Kipur?

A. See question 1459 above if Yom Kipur is considered a Yom Tov or no. Chassam Sofer would greet his talmidim after Maariv on Kol Nidrei night with; Gut Yom Tov (Darkei Chaim Vesholom (753), see also Nitei Gavriel 32: 1)
Horav Shlomo Miller’s Shlit’a opinion is that it is better to say a “Gut yohr” or “Gmar chasima tova.”
Horav Aharon Miller Shlit”a pointed out to Mishna Berura (623: 12) and Shaarei Tzion (ibid.:16) mentions that one should greet another with Gut Yom Tov at the end of Yom Kipur day, when they would make a customary seuda.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a