One of the requirements to unlock the outfit “US Army Uniform” is to eliminate all the players in a game of Poker in Blackwater. There are typically 4-Players each with $1000 you need to beat.
An easy way to do this is to scare everyone away from the table then walk away. When they respawn it should only load 1-Player, if not reload and try again.
Once you have just 1-Player simply fold until you have a decent hand and then play it. Try not to bet too much or your opponent will immediately fold, instead increase the pot slowly so that they do too. This method took me about 15 minutes.