Gone is a past participle of Go. And here is used in a 'passive' sense. Compare He has gone to the market (focus on him going) He is gone (focus on not being here, missing from us) "I have been gone " is using the perfect aspect with the present, so would be describing time up to and including now. "He has been gone to lunch for 20 minutes" is possible. Actually the error is "20 minutes ago" which is finished time being used with a present time (He has...). So it is difficult for us to at once include and exclude present time.

You are right. This sentence makes no sense.

I agree. Using "gone" in this instance complicates the phrase, but it is commonly used here in informal conversation. I wish I knew why. Simply saying "He has been on lunch for 20 minutes" is the same as saying "He started his lunch break 20 minutes ago" There are so many ways to say the same thing in English that it can get very confusing for learners. So if someone asks where an individual is, you could easily say "he went to lunch 20 minutes ago"

The sentence makes sense, but it is grammatically incorrect. If someone said it, I would assume that they meant "He went to lunch 20 minutes ago." (or "He has been gone for 20 minutes.") But don't copy that sentence.

This goes to prove that just because you see something written down it does not necessarily make it perfect, or even good, English.