Red Dead Redemption 2’s Guarma is supposed to be a temporary location, but it may be possible to return there even after the story moves on. Guarma plays a big role in Chapter Five of Red Dead Redemption 2's story, as the main characters become stranded on the fictional island. Fleeing the U.S. by boat after a bank heist gone wrong, the Van der Linde gang is caught in an ocean swell and tossed onto the tropical island's shore. But once players escape the island, it appears to no longer be accessible on the regular map.
https://screenrant.com/red-dead-redemption-2-guarma-return-how-back/