At least for Nix itself, that's pretty much it except via Dutch.

> The name Nix is derived from the Dutch word niks, meaning nothing; build actions do not see anything that has not been explicitly declared as an input

From page 81 of the original paper: https://edolstra.github.io/pubs/nspfssd-lisa2004-final.pdf

One team member is named Sönke Hahn, so it seems likely.

"gar" is a useful amplifying prefix in German that can be used in all kinds of situations and I think it lacks a direct equivalent in English. Unlike totally, for example, gar can only stand alone in very specific contexts and usually is used more like an intensifying prefix.

So garnix would be the total and utter nothing.