adverb
Emphatic adverbial intensifier, stronger than ごく alone. Used before adjectives and adjectival nouns to mean 'extremely' or 'very'.
この問題はごくごく簡単だ。
This problem is extremely easy.
ごくごく普通の一日だった。
It was a very ordinary day.
非常に is a formal intensifier meaning 'very' or 'extremely', while ごくごく is more colloquial and emphatic.
Reduplication of ごく (極), an adverb meaning 'very' or 'extremely', for emphasis.