That sense needs to be in a different entry; they can't be merged if they are from different sources. In fact the entry is there already - #2834415 (瓦落 （がら） (ateji) (n) (uk) sudden fall in stock prices; stock market 
crash)

@@ -14,5 +13,0 @@
-</sense>
-<sense>
-<stagr>ガラ</stagr>
-<pos>&n;</pos>
-<gloss>massive drop (esp. in stock value)</gloss>

***Realized がら落ち is already in JMdict.　The origin of the term as coming from ナイアガラ also seems inaccurate, as a separate finance source (posted at the bottom of this comment) lists the origin as coming from ガラガラ (1003290). It sounds probable to me that this is the actual origin and the Niagara Falls association came later.

That being said, ガラ on its own in Katakana does show up quite a bit, as well as the verbified ガラる. Perhaps a separate entry may be warranted? 

Here is the other source I mentioned: https://www.ifinance.ne.jp/glossary/souba/sou362.html#gsc.tab=0

https://www.daiwa.jp/glossary/YST0317.html#:~:text=市場全体が全面的,最大級の表現です。
https://moto-neta.com/net/gararu/
https://www.smd-am.co.jp/glossary/YST0317/
https://cisco-ccna.com/post-2824

Added second definition: ガラ to mean "massive drop".

This term comes from the last two characters of ナイアガラ and is most commonly used to describe a steep decline in the value of a stock or cryptocurrency, likening it to Niagara Falls.

A verb version "ガラる" exists as well, also mentioned in two of the sources listed.

@@ -13,0 +14,5 @@
+</sense>
+<sense>
+<stagr>ガラ</stagr>
+<pos>&n;</pos>
+<gloss>massive drop (esp. in stock value)</gloss>

I see Daijirin has it from French, but I bet the Japanese got it from English.

daijr/s
G n-grams:
ガラ	594155
ガーラ	37531