Double or Nothin'G 20

You can unlock this as soon as you finish the Flight School, which takes place at the airplane graveyard in the desert, and is part of the main storyline.

You'll get a call from Woozie, the blind Triad you helped out in San Fierro, who's taken up residence in the Four Dragons Casino at the south end of the strip in Las Venturas.

As soon as this happens, you'll have access to your first casino, and thus access to your first opportunity to unlock various gambling related achievements.

First, let's max your gambling skill to 100% to make getting the related achievements easier:

• Save outside the Four Dragons Casino and head inside
• Find the Video Poker machine on the left near the Wheel of Fortune which takes $10k bets
• Increase the wager to five times the stake value ($50k)
• Play the poker game repeatedly until you get the notification you've reached "Whale" level, which allows you to loan up to $1m from the casino
• Go back outside and save your game, to retain your maximum gambling skill

Now we can get on with the rest of the achievement!

How to unlock:

Head to any roulette table that's being attended by a croupier, move the cursor over either RED or BLACK option fully, and place the MAXIMUM BETTING AMOUNT on it. If you win, the achievement unlocks. If you lose, either reload your save or simply try again if you have the cash to spare.