There are 6 gang hideouts in Red Dead Redemption 2. You need to discover them all if you want to complete the game in 100%. This page contains information regarding all of the gang hideouts found in RDR2.
4 gang hideouts can be unlocked during the main storyline. The other 2 are in New Austin, the southern part of the map. However, these two gang hideouts are available only in the epilogue.
Gang hideouts unlocked during the main storyline
Six Point Cabin - unlocked automatically after completing Paying a Social Call main mission.
Shady Belle - after The Battle of Shady Belle main mission.
Beaver Hollow - this hideout is unlocked at the end of chapter 4, after That's Murfree Country.
Hanging Dog Ranch - this gang hideout is unlocked automatically during chapter 6.
Additional gang hideouts available in the epilogue
The epilogue is unlocked after you beat the main storyline of RDR2. During this part, you get access to the southern part of the map - New Austin. There, you can find the remaining 2 gang hideouts.
Thieves Landing - this hideout is in the eastern part of the region. Above San Luis River.
Fort Mercer - this hideout is in Big Bravo region, east of the post office.