Understanding davening: Geshem or Gashem

We all just started saying משיב הרוח ומוריד הגשם, should we say geshem or gashem? R' Yaakov Kamenetsky says that it should be geshem. He explains as follows. When you have 2 סגול at the end of a word, when that word is the end of a sentence or has a hard stop (אתנחתא) the first סגול becomes a קמץ, for example derech becomes darech. Therefore for משיב הרוח ומוריד הגשם we need to see if there is a stop after גשם or not by analyzing the second beracha of שמונה עשרה.

The second beracha is גבורות and starts with the ultimate גבורה, resurrection of the dead, then continues on to more mundane matters and then concludes with resurrection of the dead.

The initial phrase אתה גבור לעולם ה' מחיה מתים אתה רב להושיע is the introduction with states the ultimtae גבורה, resurrection of the dead. Then we have משיב הרוח ומוריד הגשם and then מכלכל חיים ... סומך נופלים רופא חולים etc., all more mundane physical things. It is clear that משיב הרוח ומוריד הגשם belongs with these physical matters, and not with תחיית המתים. Therefore there should be no stop after משיב הרוח ומוריד הגשם but rather it continues right into מכלכל חיים. If so, then the word should be pronounced geshem and not gashem.

If this is correct how did so many people come to say gashem? The answer is that in תפילת גשם the chazan says ואתה הוא ה' אלוקינו משיב הרוח ומוריד הגשם and there he says gashem because it is the end of a sentence. Since people hear the chazan say gashem in תפילת גשם they assumed that that was the correct pronunciation in davening as well.

RHS brings down in Nefesh Harav that RYBS also said geshem, I believe for a similar reason.