What is the etymology of がらんと, an adverb meaning empty or deserted?
The word is garan. The to is grammar.
There are two general things to keep in mind about 擬音語 and 擬態語:
These are general ideas and may not always work, but it is an OK start. As such, you should expect garan > karan > karakara. This may easily be verified in a dictionary.
As for the meaning, garan has two main senses: 1) a loud resounding echo when two hard (generally metallic) objects rub against each other 2) (as already given) empty, barren. The second meaning is an extension of the first in that the sound reverberates outwards spaciously.
As for karan, it two has two sense: 1) the resounding echo when two hard (generally metallic) objects rub against each other; not as loud as in garan 2) empty spaciousness, but unlike garan the sense of emptiness and loneliness is not as strong.
As for karakara, it only has the one sense: resounding echo when two metallic objects rub against each other.