https://www.gamespot.com/articles/red-dead-redemption-2-weapon-now-live-in-gta-5/1100-6460932/

Rockstar has announced another cross-promotion weapon--the Stone Hatchet--that's unlockable in both Grand Theft Auto 5 and Red Dead Redemption 2. The weapon was originally uncovered by the Guru Team, who discovered the weapon while datamining in GTA 5. If you want the Stone Hatchet in RDR2, you'll have to earn it by playing GTA Online.

The Stone Hatchet is unlocked in GTA Online after completing a series of bounties for Maude Eccles. The next time you jump into GTA Online, be on the lookout for a text message from Maude. Retrieve all the bounties--dead or alive, it doesn't matter--and you'll earn the Stone Hatchet, along with a new Kills Challenge. Completing the challenge nets you a $250,000 in-game bonus and unlocks the Stone Hatchet in RDR2.