Rockstar is really going hard this December with their content. Following The Doomsday Heist, a surprisingly huge DLC, they've now added a cross-promotion with the upcoming Red Dead Redemption 2.
These kinds of cross promotions used to be fairly common in games, however in recent years they've been toned down and mostly only appear in the fighting genre, though another more recent example is the cross promo between Final Fantasy XV and Assassin's Creed Origins. Bioware fans are also familiar with this concept, as unlocking the Blood Dragon Armor DLC for Dragon Age: Origins unlocked a similar armor suit in Mass Effect 2.
The object of the promotion at hand is a golden retro Double-Action Revolver. If you unlock it now in GTA Online by completing the treasure hunt, you'll also get it in Red Dead Redemption 2 and presumably in Red Dead Online, provided you use the same account for both.
Once you've got your hands on the gun, there is also a special Headshot Challenge in Freemode, the completion of which will grant you GTA$ 250,000, and the weapon for use in Red Dead Redemption 2. The challenge itself is really simple — get 50 headshots with the pistol. Any targets count, be they players, NPC enemies, animals or even PEDs.