가을 (fall, autumn) is an example of noun formation from a verb, albeit one which is no longer in use. The noun suffix -을 is quite common and will be seen in several other posts on this blog.
In this case, the noun 가을 comes from a now-defunct verb whose modern spelling would be 갓다. 갓다 means "to cut, slice" (cf. 긋다) and refers to fall as being the season during which crops are cut and harvested. Over time, the "ㅅ" in 갓-다 dropped when combined with the noun suffix -을, resulting in the word 가을 (갓+을 -> 가슬 -> 가을).
백문식. 우리말 어원 사전. 1st ed. 서울: 도서출판 박이정, 2014. Print.
For the original spelling of 갓다, please visit the following link (may require a plug-in for old Korean):