Geshem or Gashem?

 Igros Moshe (OC IV #40.15) Question How to pronounce “morid haGeshem” is it with a segel (geshem) or kametz (gashem)?  Answer: It needs to be said with a kametz under the gimmel (gashem) since it  is the end of the sentence and this is halacha l’Moshe M’Sinai 

Rabbi Yehuda Spitz (Ohr Someach) Several other authorities, including the Vilna Gaon (see Ashrei Ha-Ish 20:30; however, Tefillah Kehilchasah writes that there is a kabalah from the Gra that he said geshem), the Netziv (see Teshuvos Vehanhagos 2:58), the Chafetz Chaim (cited in Ashrei Ha-Ish 20, 30, quoting Kovetz Mevakshei Torah Vol. 43 p. 57), Rav Aharon Kotler, the Shaarim Metzuyanim B’Halacha, and Rav Yosef Shalom Elyashiv (Tefillah Kehilchasah 12:51; this is known as a kalballah from the Leshem), agree with this analysis. This is also how the word is presented in the siddur of the Arizal. Several other contemporary authorities, including Rav Yakov Emden, Rav Yakov Kaminetsky Levushei Mordechai, the Steipler, Rav Shlomo Zalman Auerbach, the Minchas Yitzchak, and Rav Moshe Sternbuch rule according to this custom.