A word imitative of sounds is also called gati when it is not followed by the word iti,
The phrase अनितिपरम् is a Bahuvrihi compound; अन्-इति-परम् = 'that which has not the word इति after it.'
As खाट्कृत्य having made the sound khat. खाट्कृतम् 6.2.49, यत्खाट्-करोति 8.2.71.
Why do we say 'when it has not the word इति after it?' Observe खाडितिकृत्वा निरष्ठीवत्.,