participial phrases - In "Nobody was surprised at John being absent", is "being" a present participle modifying "John" or a gerund whose subject is "John"?

Some time ago I learned the difference between a present participle and a gerund, so today I decided to pass any online test to make sure I understand it.

I passed it having made only one mistake, which asked the difference between the two in this sentence:

Nobody was surprised at John being absent.

One needed to choose between present participle and gerund in reference to the word being.

I chose present participle because the word being here plays a role of an adjective apart from a verb.

I thought that if it had read John's being, then the word would have been a participle because it would be a noun in a form of a verb.

I know, this question is a duplicate and I agree that it should be closed, but I just would like to find out whether it was me who made a mistake or whether it was the website that diddo.

Answer

It's a gerund. A gerund functions as noun. A noun is a thing. "John being absent" is a thing. It is the thing that nobody was surprised at.

It's not functioning as a participle. A participle relates to a linking verb. The only verb in the sentence is "was." The subject of that verb is "nobody." However, "nobody" wasn't "being absent."

It's also not functioning as a participle that is acting as an adjective. When a present participle is being used as an adjective, it is done so like in the following: "The running car overheated." "Being" isn't describing John like "running" is describing car. One way that you can tell is by simply moving "being" before John. Whereas sometimes adjectives can follow a noun, they usually appear before the noun they modify and can always be moved before it and still have it make sense. "Being absent John" doesn't work, so "being absent" is clearly not adjectival, so "being" is not a participle standing in for an adjective.