がちがちchattering (teeth), (frozen, worried) stiff, rigid (personality, thinking, etc.), inflexible, overly serious, voracious (e.g. work, study), without slack or pause

寒(さむ)くて歯(は)がガチガチと震(ふる)えています。

It's so cold that my teeth are chattering. {chattering (teeth), (frozen, worried) stiff, rigid (personality, thinking, etc.), inflexible, overly serious, voracious (e.g. work, study), without slack or pause}

がちがち勉強(べんきょう)したので志望校(しぼうこう)に受(う)かりました。

I studied voraciously, so I was accepted into my desired school. {chattering (teeth), (frozen, worried) stiff, rigid (personality, thinking, etc.), inflexible, overly serious, voracious (e.g. work, study), without slack or pause}

ゼラチンを入(い)れすぎたらゼリーがガチガチに固(かた)まった。

If you put too much gelatin, the jelly will become frozen stiff. {chattering (teeth), (frozen, worried) stiff, rigid (personality, thinking, etc.), inflexible, overly serious, voracious (e.g. work, study), without slack or pause}

道路(どうろ)の表面(ひょうめん)が氷(こおり)でがちがちだから滑(すべ)らないように気(き)をつけねば…。

The road surface is frozen stiff with ice, so I must be careful not to slip... {chattering (teeth), (frozen, worried) stiff, rigid (personality, thinking, etc.), inflexible, overly serious, voracious (e.g. work, study), without slack or pause}

ガチガチに硬(かた)かった芋(いも)が、電子(でんし)レンジで十分(じゅっぷん)にしてホクホクになりました。

The potato that was frozen stiff became fluffy after being sufficiently heated in the microwave. {chattering (teeth), (frozen, worried) stiff, rigid (personality, thinking, etc.), inflexible, overly serious, voracious (e.g. work, study), without slack or pause}