An onomatopoeic adverb describing doing something vigorously, roughly, or briskly — e.g., scrubbing, working out, or eating with force. Often used as "ガシガシやる."
彼はガシガシ本を読んで試験勉強をしている。
He's cramming/studying hard by voraciously reading books.
床をガシガシ掃除したら、汚れが取れた。
I scrubbed the floor vigorously and the dirt came off.
彼女はガシガシ筋トレをしているので、とても体力がある。
She trains hard with intense workouts, so she has a lot of stamina.