Alligators are one of the most solid confirmations in the marketing for GTA 6 so far. They show up fleetingly in Trailer 1 and play a big role in the wetlands sequence of Trailer 2. The model looks highly detailed, and the framing in Trailer 2 suggests active behaviour rather than them just being background props.
We consider alligators a verified wildlife species, with their mechanics still unknown. To see the full list, check the wildlife list. The region is covered in the Everglades guide.
Where alligators appear in the trailers
Trailer 1 cuts to a brief shot of an alligator on a road - short, but clearly a real animal model, not a billboard. Trailer 2 goes farther and fills out the wetland sequence: alligators near airboats, alligators close to stilt houses, alligators in shallow water. That cluster of shots lines up with an active wildlife system.
Each trailer depicts alligators only in wetlands. You never see them in urban Vice City scenes, which is accurate to life - alligators aren't urban animals.
What we can reasonably infer about behaviour
Frames from Trailer 2 seem to indicate that alligators respond to movement near them. It remains to be seen whether that means full RDR2-style predator AI or more of a lightweight ambient menace. Rockstar set a precedent in RDR2 of giving predators purposeful AI - stalking you, laying ambushes, reacting differently to your actions. The animal model and how it's framed here suggest something at least on par with that, not a downgrade.
Not shown: hunting interactions, taming, or mountable animals. Beyond ambient predator behaviour, we can only guess.
What we don't know
If alligators can be hunted. If they wander outside of the wetlands. If they can attack NPCs as well as the player. If storms or weather affect their behaviour. If the wetlands will have just a handful or an actual population.
These are standard pre-launch unknowns. Trailer clues point to a serious wildlife system. The specific mechanics won't be known until launch.