To reach the rank of Gangsta you will need to be rank 26 in any gamemode, once you reach this rank the achievement will unlock at the end of the next match. You will need 4 people minimum to start a match.
To boost rank you will need to start a match with 4 people, TruSkill is based off your games played/kills/wins etc so the more you lose the harder it will become to level up. Because of this it is best if everyone has a dummy account they can use when it is not their turn to win so their stats are not negatively affected. This is not required however. If you can find someone who is already Kingpin to lose against you this will speed it up dramatically.
When the match starts the teammate of the person getting the achievement will need to quit out. This is because the player ranking up will get a bonus for winning with uneven teams. Then the person ranking up will proceed to win the match. As soon as one person has Kingpin they will then be on the losing side which will rank the other team up quicker. If you want to check your current rank you can by returning to the multiplayer menu and the sub menu statistics. Then click on the game mode you were playing and it will show you your rank.