How can I oppose my advisor granting gift authorship to a collaborator?

I would start by asking your professor, in a non-confrontational way, why he did this. Perhaps there is some reason that is not obvious to us. Maybe he felt that the paper had a better chance of being published if this third person was involved. (Not saying that he was right to do this, just that he may have had some reason.) You might say something like this:

Professor X, as you know, I'm pretty new to academic publishing, and I'm trying to understand the process better. I was under the impression that only people who contributed to the research should be listed as authors. I'm curious why Joe Smith was added as an author. Did he make a contribution I don't know about?

If you think you may publish more papers with this professor, discuss your expectations about authorship with him and come to an agreement.

EDIT: Several people have suggested that the phrasing above is still too confrontational. I think they have a point, but I'm not sure we can come up with a phrasing that everyone is happy with. So I suggest that the OP read the comments below for ideas on how to phrase this.

Sadly, I'm going to give you advice that you will find unsatisfying as would many others. It can be dangerous to your future career to push too hard against an advisor who is behaving badly but has power over you and your future.

No matter what you do, you aren't going to change "the system." You might be able to force an editor to correct authorship to your liking, but it could be at the cost of poor, even negative, recommendations from the advisor and others in his sphere.

Make it your goal to graduate successfully and move on to another position in which you have some control over your future. This is what you lack at the moment, so I advise not making it worse. Don't think of this one paper as the only one you will ever write or that your whole future depends on it. Especially don't think that your whole future depends on getting sole authorship for this paper. You could win that battle and lose the war.

As we see in other questions here, students get stuck in such situations all too often. But the system doesn't change when it is dysfunctional and has a lot of momentum. Find a way to look to the long term and not the short. In particular, that means getting a good letter of recommendation, even if you have to bite your tongue.

Sorry that this feels wrong. But you have little power and no authority to change it.

I am a bit surprised by all these indirect answers. To my opinion you have the right to know. Therefore, write a short email (if it is a common way to communicate in your group), you can combine with other points too.

Just a naive question - what exactly was A.B's contribution to this paper?

It could be that the idea of the whole work belonged to that person.