I wonder if “calling” is gerund or anything.
If “calling” is gerund, I wonder why “calling” is gerund.

You’ve got a lot of nerve, calling me at this time of night.

I’d call it a present participle introducing a non-finite adverbial clause.

In this case, it cannot be a gerund, since it’s not a noun. They call it the adverbial participle (it answers the question “Doing what?”).
It does smack of a present participle, though.