very small fraction; tiny part; very small proportion
Emphasizes that only an extremely small portion of a whole is involved. Often used with ～に過ぎない or ～しかない.
応募者のごく一部しか選ばれない。
Only a very small fraction of applicants are selected.
この問題を理解しているのはごく一部の人だけだ。
Only a tiny proportion of people understand this problem.
わずか is an adjective/adverb meaning 'a little' or 'slight', but ごく一部 is a noun phrase focusing on a tiny subset.
Standard spelling using the adverb ごく (極) and 一部 (one part).
Kanji spelling for ごく; less common but seen in formal writing.
Combination of the adverb ごく (極, 'extremely') and the noun 一部 (いちぶ, 'one part' or 'a portion').