If you plan on doing end game raids, you shouldn't be doing them solo. I would hope by the time you reach the end game content you have found some friends your level or a guild with players your lvl to party up and do those raids. With that in mind, I would suggest three subs: Gren, Wiz and Sorc.
Pros of the gren:
3 combo abilities: 1 AOE, 1 KB (10m), 1 Stun.
Additional range dmg skills w/ slight AOE capabilities, two additional types of utility : Turret and Healing crystal and a Crit party Buff as well. Your CD's with Gren will be x3, but your subclass skills would only be used to augment your main's and as fillers for your main combo. Nonetheless your three main dmg skills as a Gren are on a relatively shorter CD than other useful sub skills running at 9 s, 12 s, 12 s. You also gain a ranged Immobilize at a hefty 36 s CD.
No class combo skills.
Additional single target ranged dmg skills on a relatively short CD of 6 s, 12 s. One which grants a - move speed as well. You then gain another line 'aoe' skill, an AOE immobilize, and a HoT skill, AOE, Gap closer, and Crit Dmg Buff.
You gain a lot more aoe dmg potential with a wizard sub, and the utility of a gap closer that stuns provides some great possibility to combo with setting a trap on stunned person/mob then having them activate the trap as well after stun. With your ultimate skills, you will also be able to stack a decent amount of -def with your skill usage.
10% dmg absorbtion every 9 s as a heal.
DoT with increased dmg received can be a constant debuff lasting 12 s, on a 6 s CD.
Leech ability constantly lasting 12s on a 12s CD.
AOE HoT with a 6s downtime.
+Dmg Party buff.
As a Gun/Sorc, you will be able to in theory do a decent job solo'ing bosses if you keep up your DoT, leech, and use your dmb asborbtion skill in rotation with your core main Gun skills. You have a HoT you can use to help you out, and then you are on your own for 6s before you can use it again. But overall a Gun/Sorc gives you an unorthodox way of surviving by maintaining your dps, and attempting to keep your dmg received to a minimum.
I have personally only have tried Gunslinger/Wiz and I like the damage I am able to do and how fast I am able, I am able to max my speed with my 50 gun buff, and have additonal food items that +speed to keep me at max if my speed buff doesn't come into play.
Overall though, Gunslinger is your main class, you are a very good single target ranged DPS and that will be your role in parties, your sub class should help your perform that role, you shouldn't chose a sub class so you can try and be that class as well, because when you have another char who's main is your sub, your sub class skills won't even affect the boss except for the dmg component.
Hope this helps with your choice