An adverbial mimetic meaning 'substantially' or 'to make a big profit' (colloquial; often about money).
彼は新しいビジネスでガッポリ稼いだ。
He made a lot of money with his new business.
投資がうまくいってガッポリ儲かった。
The investment paid off and I made a hefty profit.
会社はガッポリと利益を確保した。
The company secured a substantial profit.