adverb, adverb taking the 'to' particle
Colloquial, onomatopoeic adverb describing doing something in large amounts, heavily, or without restraint. Often used with eating, drinking, earning, or spending.
I was hungry, so I ate a big meal.
今日はがっつり肉を食べたい。
I want to eat a lot of meat today.
I hear he earns a great deal.
adverb, adverb taking the 'to' particle
Colloquial, onomatopoeic adverb meaning doing something properly, thoroughly, or to a sufficient degree. Often used with studying, working, or resting.
I have to study properly before the exam.
I slept fully over the weekend and refreshed myself.
I plan to rest properly this weekend.
I earned a lot of money from my part-time job.
I studied hard before the exam.
少し means 'a little', the opposite of がっつり's sense of doing something heavily or in large amounts.
がっちり sounds similar to がっつり but describes something sturdy or firmly built, not doing something in large amounts.
Standard hiragana spelling for this colloquial adverb.
Katakana spelling often used for emphasis or in casual writing.
Onomatopoeic/mimetic word; the exact derivation is uncertain. The form がっつり likely developed from the sound-symbolic element がつ (as in がつがつ, 'hungrily') with the adverbial suffix り, emphasizing a vigorous or hearty manner.