Why use the word "chasm" instead of just "gap" or "hole"

Well they do have similar meanings.

chasm
A deep fissure in the earth, rock, or another surface.
-- Google Dictionary

gap
A break or hole in an object or between two objects.
-- Google Dictionary

So yes, very similar. However, one of the (amazing) things about English is that there are many words that mean the same thing. A chasm is a much better word to describe a large, deep gap in the earth's surface, or an ice plane. Just reading gap, gap, gap gets boring.

So sure, you could change chasm for gap, but why would you? English isn't there to be simple, it's there to be able to describe one things is masses of detail.