Red Dead Redemption Gun in GTA Online.
You should receive an email in-game that gives a clue to help start The "Mission", this mission is a scavenger hunt that takes you all across San Andreas and you need to take pictures of different landmarks.
Doing this will unlock a free new gun to use in GTA Online, Rockstar has revealed it is the Double-Action Revolver.
Once you get your hands on the Double-Action Revolver you can enter in a Headshot Challenge during Freemode.
If you can complete it successfully, you wil get $250,000 in GTA Online and unlock the Double-Action Revolver for use in Red Dead Redemption 2.