我觉得用动名词也不能算错。确实，正如bretzhang老师所说，在通常情况下，to be done 指一次动作，doing 指习惯性动词。但要注意“通常”二字，它说明此用法并不是强制性的。比如以下句子均表示一次性动作，但用了动名词：

It's nice seeing her again. 再次见到她很好。（《张道真英语语法》）

Buying this house was a disastrous step：it’s going to have a main road built behind it. 买下这所房子是一大失策，因为房后就要建一条公路了。（《牛津高阶英汉双解词典》）

To be offered a place at such a good university is quite an achievement. / Being offered a place at such a good university is quite an achievement. 的意思是：被这样好的大学录取是件很了不起的事。