First, you should differentiate between Gerun and present participle and their usages.
You have to make sure that gerund acts as noun ( a noun that contains action and a matter of processing and accomplishing)
It could act like a subject:
- playing subject soccer with Messi and Cristiano is one of my dreams.
As a complement:
- what I want from you this year is achieving as many goals as you can.
As Object of a verb :
- I recommend reading books is the best way to improve your English.
As Object of a preposition:
- He accused her spouse of murdering his little daughter.
Part of a compound noun:
- We had no drinking water left.
Here in this example you should be wary to
comprehend this common confused issue.
Here drinking water means “the water that is used for drinking”
And another is
- Teguila drinking people are exposed to sudden death.
Here drinking acts like present participle “people who drink tequila”
And present participle acts like :
As continues action/ verb:
- what are you trying to say now
- recently, I have been playing guitar.
Acts like adjective
- You have to find attracting, interesting, exciting place to go on vacation.
As a relative clause:
- the man slaughtering/ killing people down the street was shot to death by police.
Finally, Sometimes it is a matter of interpretation whether an -ing form is a gerund or a present participle:
- Hunting poisonous snakes can be dangerous.
Hunting as a present participle functions as an adjective and describes poisonous-snakes. The sentence means:
Poisonous-snakes that hunt can be dangerous.
If hunting is a gerund, poisonous snakes is its object and the sentence means:
It can be dangerous to hunt poisonous snakes.
In your question approaching acts as a present participle.
If we reconstruct the sentence this way, it would me more clear to understand:
- approaching storm has been seen by someone.
Which voluminously means : the storm [which was approaching] has been seen