GTA Online will be getting the Doomsday Heist today; and apparently, according to what some datamining of the update’s files has found, it seems like eventually, you will get a gun in it as a reward that is actually out of Red Dead Redemption 2– the description for the gun specifically mentions the name, and also mentions that unlocking it in GTA Online will give you a special variant in Red Dead– suggesting that there will be crossovers of some sort between the two games going forward.
Whether they will be as minor as this remains to be seen- however, for Rockstar, who will now have to maintain two GaaS style games, crossing them over is an easy way of keeping both communities engaged (as well as to slip in some smart cross-promotion, too).
As for GTA Online, let’s hope Rockstar makes this gun officially and publicly available in game sooner rather than later.