Data miners recently revealed a new cross-promotion between Red Dead Redemption 2 and Grand Theft Auto Online by digging through the game files of the newest DLC of the latter game, and now Rockstar has triggered said event.

Players can unlock a new melee weapon, the Stone Hatchet, in both games after completing a short event.
GTA Online players will be contacted by Maude Eccles, who returns from GTA 5's story mode, to hunt down a series of targets. This hearkens back to her role in the story, where she provides protagonist Trevor Phillips with optional bounty missions.
Once players complete all the bounties, she provides them with the location of the weapon, allowing them to unlock it in GTA Online.
To unlock it in Red Dead Redemption 2, however, you'll need to go the extra mile and undertake the Kills Challenge. Upon getting 25 kills with the Hatchet, you earn GTA$ 250,000 and unlock the weapon in Red Dead Redemption 2. Too easy right!?
In order to actually make use of the unlock, you need to make sure you link up with the same Social Club account when it comes to playing Red Dead Redemption 2. The same is true of the other cross-promotion weapon, the Double-Action Revolver, though in that case you only get a weapon skin.
Speaking of, the Double-Action Revolver's scavenger hunt event is coming back to GTA Online alongside the Stone Hatchet, so if you missed out on that before, now is your chance to unlock it. It too comes with a GTA$ bonus, and both challenges count PED kills making them rather trivial.
The Stone Hatchet also comes with a special enrage ability, giving the player a damage reduction and health regeneration buff. This lapses quickly, however racking up kills will extend it, allowing you to go on a rage-filled spree.
Whether this ability will carry over to Red Dead Redemption 2 is unknown for now, but it would certainly make using the Hatchet a whole lot of fun and provided this really is the only way to unlock it, such a strong ability would truly make it feel like an 'exclusive' item.
We're guessing this won't be the last cross-promo between the two games, so keep an eye out for future discoveries.