You cannot legally download the licensed master recordings of "Nuthin' But a 'G' Thang" for free just because it was in a video game. Rockstar Games paid for the license to use the song inside the game, not for you to own the MP3 forever.

"Hollywood Swinging" (Kool & the Gang), "Let It Whip" (Dazz Band) Classic Country

Now, hit the road, CJ. Turn up the bass on "Hollywood Swinging," and remember: "F ck the playa if they can’t take a joke, and stay tuned to CSR, 103.9… The rhythm of the city."*

It is important to note that many modern re-releases of the game, including the Definitive Edition