看了Expert 的解答后，我觉得有几个问题，需要澄清一下：

1. There has been ...，在没有跟“一段时间”连用的情况下，不能理解为“从过去到现在一直都存在有某人/某物”，而是“曾经有过...”。

这里的完成时There has been 跟一般过去时There was， 没有本质的区别，都是指过去发生的情况。只是前者强调了对目前事件的影响而已，所以，原句完全可以改为：

 There was a gentleman asking for you. 有位先生曾来见你。

2. 分词的完成式，无论主动（having done）或被动（having been done），都不能用来作定语。我们不能说：

The boy having broken the window should be punished.

同理，也不能说：

There has been a gentleman having asked for you. 

这时，应该使用定语从句：

The boy who has broken the window should be punished.

The boy who broke the window should be punished.

3. 在There has been a gentleman asking for you. 中，asking 不一定表进行,也可以是一般式：

There has been a gentleman who asked for you.

【继续回答 whatthat2003 】

1. 现在分词完成式（过去分词没有完成式，因为过去分词done本身就表示完成，而having done或having been done都是现在分词），主要作用是：表明其动作先于谓语动词的动作发生，用来作状语。说明这个状语的动作在时间上要早于谓语动词，这类状语一般是时间状语或原因状语。

2. 现在分词的一般式，在作定语时，往往表示一个正在进行的动作，大多数语法书都这样说，其实并不尽然，有时候不一定表示正在进行，而是表示事物的特征、常态或某个动作曾经发生过，在发生时间上跟谓语动词一致。“某个动作曾经发生过”，意思是说：asking for you 作定语，本身就可以表示已经发生过，所以不用完成式。例如：

（1）The lake lying in the west of Hangzhou is called the West Lake. 等于：

The lake which lies in the west of Hangzhou is called the West Lake.

（2）There was a dog barking in the neighborhood last night. 等于：

There was a dog which barked in the neighborhood last night.

（3）Once there was a shop selling fruits oposite the street. 等于：

Once there was a shop which sold fruits oposite the street.

以上是个人观点，你的句子跟句（2）相似。