Sniping in C2 is a real challenge, and I love it, but it is hard!
The best maps for sniping are Wall street and Skyline, I'm not going to tell you where you can find the good sniping spots, that will be up to you, but I'll give you a hint, try to keep a wall behind you so you don't get stealth killed by a random by-passer
What really svcks in the game is that when you kill someone, the killcam reveals your position to that person (killcam should be forbidden for sniper users). It's hard enough as it is already. Everyone is running around and jumping like a flea, constantly zigzagging and running behind cover or bushes, it's very hard to hit a moving target with the sniper.
My favo load-out is my DSG with the silencer on (a must have!, unlock that first) and extended mag.
As for the modules, I use nano recharge (so I don't stay visible for too long when that f***ing power source depletes), blind spot (so that you don't show up as a red dot on everyone's map when you uncloak or shoot, also a must have) and then it's up to you. Retriever is handy for collecting dog tags, but don't expect to make long kill streaks, I snipe a lot and only manage only occasionally (once every 5 matches on average) to get 7 or 8 consecutive kills. Rapid fire will help a lot, especially against moving targets, the normal fire rate is slightly too slow for comfort.
Also, since you never want to stay on the same place after a kill, you will have to move, so mobility enhance might just help you with that. Always switch to you secondary when on the move, the DSG is not a competitive gun in close encounters. The hammer does well as a secundary, but I prefer the majestic (deals more damage per shot), if you have rapid fire, also consider the Nova with silencer, it's fire rate is insane.
As far as the choice for a sniper gun goes, the DSG is far superior to the GAUSS in my opinion, GAUSS has no silencer and sincerely, you can just spot them instantly, gauss users are the first ones I take down when I know they are around somewhere.
Also, the fire rate on the GAUSS is just rage inducing slow, and it deals out far less damage than you would expect for a gun with so many downsides to it. But that's a personal choice, Gauss looks bad-ass, but that's pretty much it.