Pronunciation of Gashem/Geshem

 https://dinonline.org/2011/11/07/pronunciation-of-gashemgeshem/

Rav Moshe Feinstein (Orach Chaim 4:40:15) quotes a rule cited by several Rishonim, including Tosafos, the Ran and the Rosh, that any word before a pause (esnachta) or period (sof pasuk) becomes vowelized with a kametz (hence: gashem), instead of a segol. The example given is the word "eretz": when it is the last word in a sentence or right before a pause, it changes to "aretz."

Because the word geshem is at the end of a sentence, its proper reading is thus "gashem."

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.

Yet, Rav Yaakov Kamenetsky is of the opinion that since this part of Shmoneh Esrei is called “gevuros geshamim,” meaning strengths of G-d (plural), the mentioning of the rain should not be considered the end of that sentence, but rather the beginning of the list of various powers (making rain fall, sustaining life, and so on), especially as the falling of rain and sustaining of life are interrelated, both of them referring to providing sustenance.

Therefore, he posits that the proper reading here is “geshem,” the word maintaining its usual form. He adds that this pronunciation is found generations earlier, in the siddurim of the Shulchan Aruch Ha-Rav, and the VaYaas Avraham of Tchechnov. This is also the way the word is presented in the siddur of Rav Yaakov Emden, which is known for its meticulous grammar.

Although they do not expound on the reasoning behind their practice, several other contemporary authorities, including the Levushei Mordechai, the Steipler, Rav Shlomo Zalman Auerbach, the Minchas Yitzchak, and Rav Moshe Sternbuch rule according to this custom.