A secret mission is to be unlocked in GTA 5, which will enable the players to unlock rewards for Red Dead Redemption 2.
This mission is yet to be activated. It will involve a treasure hunt which will lead you to a location of an unmarked grave. We think that this is a reference to John Marston, the protagonist of the first game Red Dead Redemption.
The reward will be a revolver in the box of the Boles Overland Stagecoach Co. Company, which can be seen in RDR 2 (see the image above). There is a chance that if you complete a headshot challenge in GTA 5, in RDR 2 you will get an exclusive variation of the weapon.