adverb
very rarely; once in a blue moon
Emphasizes extreme rarity; stronger than たまに alone. Used in casual and neutral contexts.
彼はごくたまにしか連絡してこない。
He contacts me only very rarely.
ごくたまに外食するのが楽しみだ。
I look forward to eating out once in a blue moon.
Compound of ごく (extremely) and たまに (occasionally). The exact historical derivation is uncertain, but the combination straightforwardly intensifies the rarity.