GTA 6's day-night cycle is going to be one of the things that makes the map feel alive. Trailer footage gives us a sense of how it works.
For specific night content, see the night scenes piece.
What changes with time of day
Lighting, colour grade, NPC density, NPC behaviour, content availability. Some businesses are open during the day; some at night. Traffic peaks at rush hour. Wildlife behaviour shifts (RDR2 pattern).
GTA 6's specifics are unconfirmed. The pattern is consistent across the studio's recent work.
Trailer evidence
Multiple times of day are visible in Trailer 2 - dawn, mid-day, dusk, night. The transitions read smooth rather than abrupt. Lighting shifts naturally.
Specific sequences include: dawn drive, mid-day downtown, dusk on the strip, night clubbing, late-night driving.