1. Adverb taking the 'to' particle

   chattering (teeth)

   Onomatopoeic or mimetic word

   寒くて歯がガチガチ鳴った。

2. Na-adjective (keiyodoshi)

   (frozen, worried) stiff

   Onomatopoeic or mimetic word

3. Na-adjective (keiyodoshi), Noun which may take the genitive case particle 'no'

   rigid (personality, thinking, etc.); inflexible; overly serious

   Onomatopoeic or mimetic word

4. Adverb (fukushi), Noun which may take the genitive case particle 'no'

   voracious (e.g. work, study); without slack or pause

   Onomatopoeic or mimetic word

Also written がちがちgachigachi