As noted by insider and dataminer GameRoll, the files of Red Dead Redemption 2 contain a set of unused boat textures with various names — including "The Emily Bell" and "Moses T Prescott". However, the boat named "The Leonida" is of particular interest now.
This name directly coincides with the fictional state of Leonida, where GTA 6 will take place. Leonida is an analogue of Florida, and Vice City is an interpretation of Miami in the Rockstar universe.
According to GameRoll, there are two possible explanations for the find: Rockstar either initially planned to subtly hint at GTA 6 in RDR2, or that's where the name of the future state originated.
Rockstar has previously stated that Leonida will be a large-scale and detailed region.