To get the Red Dead Redemption 2 Double-Action Revolver in GTA Online, you need to complete a special treasure hunt event that starts with receiving an email from vanderlinde@eyefind.com titled "Treasure Hunt." This email contains a photograph that serves as your first clue to the revolver's location.

1. Receive the Email and Locate the First Clue 
   After entering a freeroam lobby in GTA Online GTA 5 Enhanced Accounts, wait for the treasure hunt email. The email includes a photo hinting at a location somewhere in Los Santos. Rockstar helps by highlighting a general area on your mini-map. You must explore this zone to find a piece of paper that matches the photo in the email. A sound cue of chiming bells grows louder as you approach the clue, making it easier to find.

2. Find Three Additional Clues
   After discovering the first clue, three more yellow question marks appear on your map. Each corresponds to a location where you must find further clues-items like a bloody shovel near Sandy Shores, a weapon case near Grapeseed, and a body in a cave near Tundra Valley. These clues lead you closer to the final treasure.

3. Claim the Double-Action Revolver 
   Once all clues are collected, a final location is revealed on your map. Head there to find a chest marked with "Boles Overland Stagecoach Co." Opening this chest grants you the Double-Action Revolver, a rare weapon from Red Dead Redemption 2 now usable in GTA Online.

4. Complete the Headshot Challenge for Rewards  
   After obtaining the revolver, a challenge appears: get 50 headshot kills on any targets (players or NPCs). Completing this challenge rewards you with $250,000 in GTA Online and unlocks a unique golden skin for the revolver in Red Dead Redemption 2, provided your Rockstar Games Social Club accounts are linked GTA 5 Enhanced Accounts buy.

The revolver features satisfying animations, including a distinctive holster twirl, adding to its appeal. This treasure hunt was originally a promotional event tied to Red Dead Redemption 2’s release but remains available for players to complete.