I'm busy sightseeing: sightseeing: noun / gerund / present participle?

[Sweetpumpkin]

Hi,

I'm busy sightseeing.

In this sentence, is sightseeing a noun, a gerund, or a present participle?

Thanks,
sweetpumpkin

[Mister Micawber]

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Well, a gerund is a noun. However this 'sightseeing' is a non-finite clause, since I cannot put a noun in its place and I cannot modify it with 'very'.

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Native English teacher at Mister Micawber's