An adverb meaning markedly or noticeably — indicating a sudden or significant change in degree or amount.
仕事の経験を積んで、給料がぐんと上がった。
After gaining work experience, my salary went up sharply.
彼の日本語力は半年でぐんと良くなった。
His Japanese improved markedly in six months.
暖房を入れると部屋がぐんと暖かくなった。
When we turned on the heater, the room became noticeably warmer.