Unconfirmed: treat this as a rumor until Rockstar confirms it.
This one is unconfirmed, but the track is already circulating, and it sounds like the real thing.
What the leak claims
According to RockstarINTEL, the GTA 6 main menu background music has leaked. It's the track that plays whenever the game's main menu is open on console. The leak arrived just hours before pre-load on PS5 and Xbox Series X|S.
Where it comes from
RockstarINTEL says the music was revealed by Razed, known for the NaturalVision Evolved and Enhanced GTA V graphics mods. A clip of the track was posted on X by @TexFunz2, with credit to @Word. RockstarINTEL also reports that the game had already been uploaded to PlayStation's store backend, which is why file-level details have been surfacing.
How credible it is
Several things point toward authenticity. The clip came with a named credit, and RockstarINTEL says fans on social media already believe it's the real score because of how it sounds.
The composer is the open question. RockstarINTEL notes it's unknown whether Woody Jackson, who scored GTA V, wrote this piece. Rockstar has not confirmed his involvement with GTA 6.
What Rockstar has said
Rockstar has not addressed the track publicly. Until it does, treat the music as a leak, not an official reveal.