Ehm, you really don't think that GTA V is the most sold game ever thanks to the brand itself being that famous? TW3 is the most awarded game in history (goty), but GTA V has sold several times more (100+ millions copies) and is the only game which has made $1 billion in the first 3 days after launch.
It's not a matter of how good the actor is or how s/he fits in the role, it's a matter of how popular the actor is. It's pure marketing. Why do you think commercials have famous people? Why are those famous people paid that much by companies to advertise their product? Why do you think "influencers" are a thing?
Does Natalie Portman smell good? Does she fit in a Dior's ad contest more than anyone else? Why this thing exist and is broadcasted on tv?
She is an excellent actress (oscar winner for best actress in 2011 and well deserved), but she's not showing any of this in the ad. She is there for the only and single reason that she is famous, very famous.
Just to make it clear, I'm contrary to the presence of lady gaga in the game (I fear more "damage" than "gain"), I'm just saying why CDPR has decided to hire her instead of any "relatively unknown" (sorry, but followers are an excellent data, used by companies themselves) actor for that specific role she's gonna play. Lady gaga will help CP2077's sales thanks to the simple fact that she is in the game. Need another demonstration? there are exactly 100 posts about this topic just on this forum, people are talking about the game. And it's just a rumor, not even confirmed. That's how marketing has always worked and always will.
Now, if you really think that marketing doesn't affect sales, I honestly give up this conversation and don't have much to add.