the hair disheveled, as in mourning
髺 centers on hair that is disheveled or unkempt, especially as a sign of mourning or grief. The idea is hair left loose and untidy rather than neatly arranged.
髺 combines 髟 (hair) with 舌, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with disheveled hair, particularly in mourning contexts.
The top 髟 shows long flowing hair, and the bottom 舌 looks like a tongue hanging out. Picture someone in mourning with hair unkempt and mouth open in grief.
For ガチ, imagine a grieving person with disheveled hair clutching a picture: the sound 'gachi' suggests a tight, desperate grip, like 'gachi' as in 'gachi gachi' (clattering or tight).