Q: Hurr durr can't get into freeroam.
A: Start a singleplayer game. From there, press start and begin a Private Free Roam from the multiplayer tab. Once the private free roam starts, press start again and choose Public Free Roam. This works around the infinite loading screen.
Q: WTF IS GOING ON IN PUBLIC FREEROAM?
A: The recent patch and addition of co-op content has broken the public freeroam, causing all NPCs, other players, and wildlife to not appear. This means no mount. Other players can still be in your session, but they just appear invisible.
Q: OMG HOW I DO DIS GLITCH?
A: Find the nearest fast travel point. No mount means you're walking. Then set a waypoint on the hideout of your choice (hint: Ft. Mercer is the best, and almost always taken already). When you arrive, you'll be prompted to clear out a nonexistent gang. Wait a few seconds, and you'll receive credit for clearing the gang and the associated XP (usually around 400).
Q: How do I maximize XP?
A: Get access to Fort Mercer and turn expert aim on. Expert aim increases your bonus significanty. Fort Mercer also has the shortest "lag" between starting the mission and receiving credit (about 1.2 seconds). If Fort Mercer is occupied, Twin Rocks is a viable alternative, granting 400 XP in 3 seconds.
This works fastest when only one person is attempting the hideout, because all players must agree to "replay" a mission in order for it to start again.
Q: How can I be a jerk?
A: Whether to exploit or stop other players, the easiest way to get people off a gang hideout is to just get there, and refuse to replay the mission. Most players will get audibly upset, calling you various rude names, voting to kick you, etc. These attempts are almost always futile, and they soon leave the session. You are now free to either exploit, or keep others from doing it.