The day-night cycle is going to be one of the things that breathe life into the GTA 6 map. We learn a bit about how it works from the trailer footage.
Night footage is broken down in the night scenes piece.
What changes with time of day
Ambient light, colour grading, NPC population and behaviour, availability of activities. Some shops are open during the day; some at night. Traffic peaks at rush hour. Animal behaviour changes (similar to RDR2).
No GTA 6 specifics have been confirmed. These traits apply consistently across Rockstar's recent projects.
Trailer evidence
Various times of day appear in Trailer 2: dawn, mid-day, dusk, night. They transition seamlessly, rather than suddenly. Lights change naturally.
Examples include: a dawn drive, mid-day downtown, sunset on the strip, nightclubbing, late-night driving.