Gunslinger is the name of the ranged DPS Advanced Class for Smugglers.  DPS means Damage Per Second.  This means their primary role is to damage enemies from afar.  This type of Smuggler is not a close quarters class.  If you like to rush into the mêlée, you may want to look at the Scoundrel or another class altogether.  If you get enjoyment from dropping an enemy before he can even reach you, then the Gunslinger is just what you're looking for.
Gunslingers have three specs or 'trees' available: Dirty Fighting, Saboteur, and Sharpshooter.  Dirty Fighting deals it's damage steadily over time, also known as DoTs.  The  Saboteur uses attacks that affect a sizeable area, also known as AoE attacks, which have the potential to affect several targets at once.  Finally, a Sharpshooter can deal a huge amount of damage to a single enemy.  Your preferred style of gameplay is the determining factor in choosing.  Let's go over the three trees to see which one might have what you're looking for.
A Smuggler using the Dirty Fighting tree deals damage to enemies mainly with bleeds (DoTs).  The damage caused by bleeds is Internal Damage.  Internal Damage has no resistances, and is the ideal type of damage.  This is because regular armor and defenses do not reduce Internal Damage.  To maintain play balance, Internal Damage done is in small doses, and over time.  It's akin to using poison to kill someone slowly, as opposed to shooting them with a blaster.  The Dirty Fighting tree also requires the most mobility of any of the trees. This is because you have to outlast our opponents, as opposed to having to overcome them immediately.  This puts you into a hit-and-run type play style, where you attack a few times, then run off to recharge and pick a new target.  Running and hiding are important strategies for this tree.
The Saboteur tree deals with Area of Effect attacks.  You will use Thermal Grenades, Incendiary Grenades, and other explosives to deal damage to every enemies within the area.  This is ideal for attacking groups like those found in PvP warzones, Heroic areas, and Quest objectives.  Gameplay balance also plays a part in limiting the Saboteur's amazing AoE potential.  While AoE attacks deal more damage overall than single target attacks, the damage taken per target is reduced.  For example, assume an AoE attack deals 1000 damage.  That 1000 damage is spread over all the enemies within the area, and not 1000 to each enemy.  These large AoE attacks also use more energy than single attacks.  This means you can do fewer, high damage attacks as opposed to many, lower damage attacks.  Cover is also important to a Saboteur.  You need to use your Crowd Control abilities when repositioning or if enemies get too close.
The Sharpshooter tree is known for its ability to dole out sustained damage.  They can deal large amounts of damage to a single enemy.  Operations and Flashpoints bosses are a Sharpshooter's bread and butter.  As with the other two trees though, there is a gameplay balance factor.  The Sharpshooter tree spends a lot of time in cover.  Because they cannot move around, they are more vulnerable than the other two trees at times.  They do have some CC abilities.  These abilities must be used wisely.  The high damage output of this tree makes it is one of the highest damage dealing classes in the entire game.
In a nutshell, the Dirty Fighting tree involves a hit-and-run play style.  Get your damage rolling, then switch to a new victim!  Saboteur gameplay depends on ensuring proper placement of your AoEs, catching as many opponents as you can.  Lastly, the Sharpshooter tree involves looking for a good vantage point to unleash volley after volley of blaster fire.  So try one of these trees out for a few days, and see how it does.  If you try one and find you don't like it, you can always switch.  There is sometimes a credit cost involved if you switch too often, but your first switch or 'respec' is free. 
Happy hunting!