@futurile I'm in favor of people calling them whatever they feel comfortable using.
But the Guix manual says that
«“Guix” is pronounced like “geeks”, or “ɡiːks” using the international phonetic alphabet (IPA).»
https://guix.gnu.org/manual/devel/en/html_node/Introduction.html
So I personally use "guix" to refer to "Guix users", and "guik" for the singular. And it works for me because it reads naturally in Spanish.