adverb, adverb taking the 'to' particle
rapidly; vigorously; by leaps and bounds; steadily
Describes something progressing quickly and energetically, often with a sense of momentum. Common with verbs of growth, increase, improvement, or movement.
この植物はぐんぐん伸びている。
This plant is growing rapidly.
彼の成績はぐんぐん上がった。
His grades improved by leaps and bounds.