Grassrivers is Leonida's swamp. Along with Vice City and the Keys, it is one of the three landscapes Rockstar has named as the backbone of the state.
The country station Back Country Radio is made expressly for this kind of water: the announcement mentions a weather report from an airboat. Mount Kalaga adjoins this kind of terrain as wilder backcountry.
Rockstar introduces the region with 'Welcome to the Wetlands' and calls it 'the untamable jewel of Leonida's crown': a primordial expanse of mangroves where alligators are not the deadliest predators.
In the Extended Look, Jason steers a boat through its waterways, and alligator hunting is among the activities shown. Fanboats are on the list of vehicles in the footage as well.
See where the regions sit on the schematic GTA 6 map.