As verbs the difference between gack and nack
is that gack is to make a 'gack' noise while nack is (computing) to acknowledge negatively; to send a nak signal to.
As an interjection gack
is (often|repeated several times) the sound of a cat coughing up a hairball.
gack
English
Interjection
(
en interjection)
(often, repeated several times) The sound of a cat coughing up a hairball.
Also used of similar noises, particularly one made to show disgust or disapproval.
Verb
(
en verb)
To make a 'gack' noise.
To expectorate a hairball.
nack
English
Verb
(
en verb)
(computing) To acknowledge negatively; to send a NAK signal to.