Was it good seeing him?
Was it good meeting him?
Is it grammatical and does it sound natural?
Both are grammatical and sound natural. There is a difference in meaning between the two sentences. "Seeing him" implies that you already know him, while "meeting him" implies that you did not already know him before meeting him.
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Both are grammatical and sound natural. There is a difference in meaning between the two sentences. "Seeing him" implies that you already know him, while "meeting him" implies that you did not already know him before meeting him.