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.
今日はがっつり肉を食べたい。
I want to eat a lot of meat today.
彼はがっつり稼いでいるらしい。
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.
試験前はがっつり勉強しないと。
I have to study properly before the exam.
週末はがっつり寝てリフレッシュした。
I slept fully over the weekend and refreshed myself.
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.