Grand Theft Auto 5 is slowly taking on the characteristics of a billboard. Right now, it is a very exclusive billboard that is hard to get onto, but it is steadily traveling down a path towards becoming an advertising platform.
So far, only Rockstar and entities in close collaboration with Rockstar have made use of the game's immense mainstream popularity to gain publicity, and we don't expect it would be Rockstar themselves who branch out in terms of in-game advertisements. And yet, when musicians debut new songs in GTA Online on the in-game radio, the implications multiply.
First, let's take a step back and look at in-game advertising as a whole.
This is far from a new concept, as free-to-play games often survive by way of in-game ads. The practice is particularly predominant in mobile and social media games, but have appeared elsewhere.
Paid games very rarely include advertising — right now only one example comes to mind off-hand, which would be the overall forgettable and unpopular Battle L.A. licensed movie tie-in FPS that had adverts. But in that case the companies advertised in the game were the same ones sponsoring the film, and these ads appeared as in-universe props, meaning actual billboards present in the game maps.

Rockstar has used GTA Online to spread awareness of Red Dead Redemption 2 several times — once with an unavoidable loading-screen prompt, and through a pair of cross promotions with gameplay content as well as linked pre-order bonuses. These are far from problematic instances, as game companies cross-promote their titles frequently. It's an interesting and engaging way to bridge communities and includes fun cross-over items or quests that service fans of both IP.
Recently, this practice expanded. With the After Hours DLC featuring real-life DJs and their actual music, a unique opportunity arose.
Tale of Us, one of the artists attached to After Hours, included some of their songs from a then-unreleased album on the new in-game radio channel, Los Santos Underground, as an exclusive preview. Essentially, the music of a famous and prominent artist debuted in GTA Online.
Ivan Pavlovich, director of music at Rockstar Games recently talked with Rolling Stone specifically about After Hours and the importance of music in GTA and in all of Rockstar's titles. He talked about how powerful a tool for music discovery GTA 5 is, as it introduces a vast amount of people to new and interesting music through its wide selection of licensed tracks and huge player base.