adverb
in great quantity; plentifully; greatly
Colloquial adverb emphasizing abundance or a large amount. Often used with verbs like ある, いる, できる, or 持っている.
この辺りには、ごまんと虫がいる。
There are tons of bugs around here.
彼はお金をごまんと持っているらしい。
He seems to have a huge amount of money.
たくさん is a standard, neutral word for 'many' or 'a lot'. ごまんと is more colloquial and emphatic, often implying an overwhelming or excessive quantity.
うんと is also a colloquial adverb meaning 'very much' or 'a lot', but it is more often used with verbs of effort or degree (e.g., うんと頑張る). ごまんと focuses on the sheer quantity of something existing or being possessed.
Derived from 五万 (ごまん, 'fifty thousand'), used figuratively to mean a very large number, plus the particle と. The kanji form 五万と is ateji.